Trotec SL400 Reader for Android

An Android application for reading measurements from a Trotec SL400 sound level meter over USB OTG / USB serial, displaying live sound values in a custom mobile UI, capturing incident audio, and optionally forwarding alerts and remote-control commands through Matrix.

This project now combines five main areas:

USB serial communication with the SL400
Reverse-engineered protocol decoding for measurement frames
Acoustic metrics and alert evaluation
Incident audio buffering and clip upload
Matrix integration for messaging, alerting, clip delivery, and remote control

Overview
Current Features
Project Status
Architecture Overview
How It Works
User Interface
Matrix Commands
Configuration
Project Structure
Requirements
Permissions and Android Manifest Notes
Build and Run
Troubleshooting
Known Limitations
Roadmap Ideas
Disclaimer
License

Overview

The goal of this app is to turn an Android device into a mobile field companion for the Trotec SL400.

The app can:

detect compatible USB serial devices
connect to the SL400 over USB OTG
continuously read and decode measurements
show the latest sound level in dB
compute rolling acoustic metrics such as LAeq 1 min / 5 min / 15 min and max 1 min
evaluate configurable alert rules
capture buffered audio clips around incidents
store and manage incident history locally
send alert messages and audio clips to Matrix rooms
receive remote commands from a Matrix room to update alert behavior, query incidents, and fetch clips

The UI is optimized around a simple live measurement screen and a separate settings area for configuration and diagnostics.

Current Features

USB / Device Features

USB OTG based device connection
detection of compatible USB serial devices using usb-serial-for-android
explicit USB permission handling through UsbManager
automatic device refresh on attach / detach
connect / disconnect support
foreground service while measurement is active

Measurement Features

marker-based protocol parsing
live decoding of SL400 frames
extraction of measurement values and auxiliary bytes
recent measurement history in the UI
rolling acoustic metrics over multiple time windows

Acoustic Metrics Features

current live value
LAeq 1 minute
LAeq 5 minutes
LAeq 15 minutes
max 1 minute
time-above-threshold tracking for the last minute
window coverage tracking so alerts do not trigger before enough data exists

Alert Features

configurable threshold in dB
hysteresis support to avoid noisy re-triggering
threshold crossing or periodic alert modes
selectable trigger metric
target room override for alert delivery
persisted alert configuration with DataStore

Audio / Incident Features

continuous audio ring buffer for incident capture
configurable pre-roll and post-roll clip creation in code
local WAV clip generation
automatic audio hint analysis with confidence, source hints, and quality flags (sub/bass/mid/high ratios, low-end pulse detection, clipping/DC/low-signal checks)
local JSONL-based incident store
retention cleanup for old incidents and clips
reuse of already uploaded Matrix media URLs when possible

Matrix Features

send Matrix test messages
send alert messages to a Matrix room
sync a Matrix command room
ignore the app's own Matrix user during sync
persist Matrix sync token across sessions
modular Matrix command handling
remote alert configuration via Matrix chat commands
incident list queries by time window
summary queries
JSON export of incident data
on-demand graphs (PNG)
daily report scheduler (summary + optional JSON + optional graph)
audio buffer remote control
clip upload on demand
audio hint follow-up messages with confidence and source hints

UI Features

custom themed live measurement screen
settings screen with separate Matrix and alert sections
USB audio test screen
alarm audio buffer screen
custom background and device imagery
recent sample preview list with a bounded history

Project Status

This project is based on a partially reverse-engineered protocol.

What is already working well:

USB serial connection
measurement frame parsing
live value display
rolling acoustic metrics
alarm evaluation logic
Matrix alert sending
Matrix command sync and parsing
incident storage
incident clip generation and upload
persistent settings with DataStore

What is still evolving:

protocol documentation for all tags
UI polish and information architecture
clip metadata enrichment such as duration and waveform previews
more advanced graphs and history views
more robust background behavior on all Android / USB combinations
broader hardware testing for USB microphones and hubs

Architecture Overview

At a high level, the app works like this:

Sl400Repository reads raw serial data from the device.
Sl400Decoder converts tagged byte frames into Sl400Sample objects.
AcousticMetricsEngine builds rolling metrics from the live sample stream.
AlarmEvaluator decides whether an alert should be fired based on the selected metric mode and alert settings.
IncidentRepository stores the incident, while AlarmAudioCaptureCoordinator creates a buffered WAV clip.
HttpMatrixPublisher sends alert messages and optional audio clips to Matrix.
MatrixSyncClient listens for commands from the configured Matrix command room.
MatrixCommandProcessor and its command handlers interpret remote commands and apply changes or queries.

The orchestration happens in Sl400ViewModel.

How It Works

USB / Serial Communication

The app uses Android's USB host APIs together with the usb-serial-for-android library.

Serial parameters currently used:

Baud rate: 9600
Data bits: 8
Parity: NONE
Stop bits: 1
Flow control: NONE
DTR: false
RTS: false

Device discovery happens via UsbSerialProber.getDefaultProber().findAllDrivers(...).

When a user selects a device:

the app checks USB permission
Android permission is requested if needed
the first available serial port is opened
serial parameters are applied
a read loop starts in a coroutine on Dispatchers.IO

Main class:

sl400/Sl400Repository.kt

SL400 Protocol Decoding

The protocol is currently interpreted as a tagged byte stream with 0xA5 as a marker byte.

The decoder works as a small state machine:

SEEK_MARKER
READ_TAG
READ_PAYLOAD

Known tags currently handled:

0x0D → measurement payload
0x06 → auxiliary 3-byte payload
0x00 → sample end / sample emit trigger
additional observed tags with fixed lengths are recognized but not yet semantically decoded

Measurement decoding currently uses a packed BCD-like interpretation:

hundreds from low nibble of byte 1
tens / ones from byte 2
final value is represented as tenths of dB

Example:

raw tenths 478 → 47.8 dB

Main decoder:

sl400/Sl400Decoder.kt

Samples and Measurement Model

Each complete sample is represented as:

data class Sl400Sample(
    val timestampMs: Long,
    val db: Double,
    val rawTenths: Int,
    val aux06Hex: String?,
    val tags: List<Int>
)

This gives the app both:

a user-friendly value (db)
a raw machine-friendly value (rawTenths)
optional auxiliary payload data
the seen tag list for debugging / analysis

The most recent samples are exposed through a SharedFlow in the repository and collected in the Sl400ViewModel.

Acoustic Metrics Engine

Rolling metrics are computed by:

sl400/AcousticMetrics.kt
sl400/AcousticMetricsEngine.kt

The engine keeps a sliding in-memory sample window and computes:

currentDb
laEq1Min
laEq5Min
laEq15Min
maxDb1Min
timeAboveThresholdMs1Min
coverage1MinMs
coverage5MinMs
coverage15MinMs

Why coverage matters

LAeq-based alerts should not trigger before enough data exists.

For example:

LAEQ_1_MIN requires at least 60 seconds of valid coverage
LAEQ_5_MIN requires at least 5 minutes of valid coverage
LAEQ_15_MIN requires at least 15 minutes of valid coverage

This logic prevents false early triggers when the app has only recently started collecting measurements.

Metric modes available for alerting

enum class MetricMode {
    LIVE,
    LAEQ_1_MIN,
    LAEQ_5_MIN,
    LAEQ_15_MIN,
    MAX_1_MIN
}

Alarm Evaluation

Alert behavior is handled by:

alert/AlertConfig.kt
alert/AlarmEvaluator.kt

Supported alert settings

enabled
thresholdDb
hysteresisDb
minSendIntervalMs
sendMode
metricMode
allowedSenders
commandRoomId
targetRoomId
alertHintFollowupEnabled
dailyReportEnabled
dailyReportHour
dailyReportMinute
dailyReportRoomId
dailyReportJsonEnabled
dailyReportGraphEnabled

Alert modes

`CROSSING_ONLY`

Sends an alert only when the selected metric crosses the threshold from below.

`PERIODIC_WHILE_ABOVE`

Sends an alert when crossing the threshold and then continues sending periodically while the selected metric remains above the threshold and the minimum interval has elapsed.

Hysteresis behavior

Hysteresis prevents repeated re-triggering around the threshold.

Example:

threshold = 70.0
hysteresis = 2.0

The system triggers above 70.0 and only resets once the selected metric falls to 68.0 or below.

Alert timing and audio hints

When incident audio capture is enabled, alerts are delayed briefly so the audio hint can be included. If the hint is not ready within a short timeout, the alert is sent without it. Optionally, a follow-up hint message can be sent depending on alertHintFollowupEnabled.

Example audio hint message:

SL400 audio hint (id=...): bass-heavy music (0.79), likely stage / PA dominated (clipped)

Incident Storage and Retention

Incident persistence is handled by:

incident/IncidentRecord.kt
incident/IncidentRepository.kt

Incidents are stored locally in:

filesDir/incidents/incidents.jsonl

Each incident stores:

incident ID
timestamp
room ID
metric mode and metric value
threshold at trigger time
rolling acoustic metrics
time above threshold for the last minute
local clip path
upload state
Matrix content URI (mxcUrl) if already uploaded
optional audio hint field

Cleanup behavior

The repository can automatically:

remove incidents older than a configured retention window
limit the total number of incident records
limit the total number of stored clips
delete unreferenced clip files after cleanup

Current cleanup call in the view model keeps:

up to 14 days of incident history
up to 2000 records
up to 200 clips

Audio Capture and Incident Clips

Incident audio buffering is handled by:

audio/AlarmAudioCaptureCoordinator.kt
audio/AudioRingBuffer.kt
audio/WavWriter.kt

What it does

The app can keep an in-memory ring buffer of microphone audio so that when an alert fires, it captures a clip containing audio from before and after the incident.

Current defaults in code:

ring buffer: 30 seconds
pre-roll: 10 seconds
post-roll: 20 seconds

Audio sources

The app tries to pick a preferred input device in this order:

USB device
USB headset
first available input

Audio controls currently available

local start/stop of the alarm audio buffer in the UI
remote Matrix commands for audio start, audio stop, and audio status
local USB microphone test recording via AudioTestRecorder

Audio hint analysis

Audio hints are computed from incident clips and include label, confidence, source hint, and quality flags (clipped input, DC offset, low signal). Hints can be computed from WAV files or directly from PCM:

AudioHintAnalyzer.analyzeWav(file)
AudioHintAnalyzer.analyzePcm16(pcmData, sampleRate, channels)

Output format

Incident clips are written as WAV / PCM 16-bit files.

Example file location:

filesDir/audio_incidents/incident_<id>_<timestamp>.wav

Concurrency guard

The audio coordinator uses an isCapturing flag so multiple incident captures do not overlap.

Matrix Integration

Matrix support is implemented as a lightweight client integration.

Main files:

matrix/MatrixConfig.kt
matrix/MatrixSettingsRepository.kt
matrix/HttpMatrixPublisher.kt
matrix/MatrixSyncClient.kt
alert/MatrixCommandProcessor.kt
alert/MatrixConfigCommandHandler.kt
alert/MatrixAudioCommandHandler.kt
alert/MatrixQueryCommandHandler.kt
alert/MatrixClipCommandHandler.kt
alert/MatrixExportCommandHandler.kt
alert/MatrixReportCommandHandler.kt
alert/MatrixGraphCommandHandler.kt
alert/MatrixHelpCommandHandler.kt

Sending

The app can send:

plain Matrix test messages
alert messages
text responses to Matrix commands
audio clips as m.audio
files (JSON) as m.file
graphs (PNG) as m.image
already-uploaded audio again using a stored mxc:// URL without re-uploading

Sync

The app can sync a command room using /sync and:

persist the next_batch token
ignore events coming from its own Matrix account
parse chat commands
update alert settings from received commands
answer queries with text summaries
upload or resend clips

Remote control model

The app listens for commands in a configured commandRoomId and uses allowedSenders to restrict which Matrix users are allowed to control the device.

Daily reports

Daily reports are scheduled via WorkManager and can send:

a summary message
an optional JSON export
an optional incident graph (PNG)

Background Service

When the app successfully connects to the SL400, it starts a foreground service:

Sl400ForegroundService.kt

This service exists to support background measurement operation and keep the app alive while the USB connection is active.

It creates a low-importance notification channel and runs with:

foreground notification
FOREGROUND_SERVICE_TYPE_CONNECTED_DEVICE

This is important for newer Android versions.

User Interface

The app currently has two main UI modes plus additional settings cards.

1. Home Screen

A simplified live measurement screen with:

large current dB value
USB connection status
background artwork
device illustration
quick actions for auto-connect and opening settings

2. Settings Screen

Structured into cards for:

USB / connection handling
Matrix configuration
alert configuration
USB microphone test
alarm audio buffer
recent samples

3. Matrix / Remote Control View

The Matrix integration lets you:

configure alerts remotely
start / stop audio buffering remotely
request incident lists
request summaries
request clips

UI is written with Jetpack Compose and themed via:

ui/theme/Color.kt
ui/theme/Theme.kt
ui/theme/Type.kt

Matrix Commands

The app currently supports the following Matrix chat commands in the configured command room:

!sl400 help
!sl400 status
!sl400 config
!sl400 set enable true|false
!sl400 set threshold 72.5
!sl400 set hysteresis 2.0
!sl400 set interval 30000
!sl400 set mode crossing|periodic
!sl400 set metric live|laeq1|laeq5|laeq15|max1
!sl400 set commandroom !abc123:server.tld
!sl400 set targetroom !xyz456:server.tld
!sl400 allow @user:server.tld
!sl400 deny @user:server.tld
!sl400 set dailyreport true|false
!sl400 set reporttime 08:30
!sl400 set reportroom !report:server.tld
!sl400 set reportjson true|false
!sl400 set reportgraph true|false
!sl400 set alerthint true|false
!sl400 audio start
!sl400 audio stop
!sl400 audio status
!sl400 incidents since 30m
!sl400 incidents between 2026-03-27T18:00 2026-03-27T23:00
!sl400 incidents today
!sl400 incidents yesterday
!sl400 clips since 2h
!sl400 clip last
!sl400 clip incident <incidentId>
!sl400 summary since 1h
!sl400 summary today
!sl400 summary yesterday
!sl400 json today
!sl400 json yesterday
!sl400 json since 24h
!sl400 json since 24h cliponly
!sl400 json since 24h metric=laeq5
!sl400 report now
!sl400 report today
!sl400 report yesterday
!sl400 report since 24h
!sl400 graph today
!sl400 graph yesterday
!sl400 graph since 6h

Command behavior

status returns the current alert configuration
set ... updates alert and report configuration
commandroom changes the command room ID
targetroom changes the destination room for alerts
allow adds a Matrix user to the allowed sender list
deny removes a Matrix user from the allowed sender list
audio ... controls or queries the audio ring buffer
incidents ... returns incident lists for a requested time range
clips ... returns available clip information
clip ... uploads the latest or a specific incident clip
summary ... returns a compact aggregated incident summary
json ... returns a JSON export of incidents as a file upload
report ... returns a summary and (optionally) JSON/graph based on report settings
graph ... returns a PNG graph of incidents

Configuration

Matrix Settings

The following settings are stored using DataStore:

homeserverBaseUrl
accessToken
roomId
enabled
syncToken

These are managed in:

matrix/MatrixSettingsRepository.kt

Notes

roomId should be the Matrix room ID used for test messages or as a default target room
HTTPS should be used for the homeserver URL
access tokens should be treated as sensitive credentials

Alert Settings

The following settings are persisted using DataStore:

alert enabled / disabled
threshold
hysteresis
minimum interval
send mode
metric mode
command room ID
target room ID
alert hint follow-up enabled
daily report enabled
daily report time
daily report room override
daily report JSON enabled
daily report graph enabled
allowed senders CSV

These are managed in:

alert/AlertSettingsRepository.kt

Allowed Senders

Allowed Senders is a comma-separated list of full Matrix user IDs that are allowed to send configuration commands.

Example:

@alice:example.org,@bob:example.org

If the list is empty, any sender in the configured command room can issue commands.

Recommended production setup:

use a dedicated command room
allow only specific user IDs
keep the command room separate from the alert room
use a dedicated Matrix bot account or app account for SL400 control

Project Structure

app/src/main/java/de/drremote/trotecsl400/
├── MainActivity.kt
├── Sl400ForegroundService.kt
├── UsbDeviceExtensions.kt
├── alert/
│   ├── AlarmEvaluator.kt
│   ├── AlertConfig.kt
│   ├── AlertSettingsRepository.kt
│   ├── JsonExporter.kt
│   ├── DailyReportScheduler.kt
│   ├── DailyReportWorker.kt
│   ├── IncidentGraphRenderer.kt
│   ├── MatrixAudioCommandHandler.kt
│   ├── MatrixClipCommandHandler.kt
│   ├── MatrixCommandHandler.kt
│   ├── MatrixCommandParsing.kt
│   ├── MatrixCommandProcessor.kt
│   ├── MatrixCommandTypes.kt
│   ├── MatrixConfigCommandHandler.kt
│   ├── MatrixExportCommandHandler.kt
│   ├── MatrixGraphCommandHandler.kt
│   ├── MatrixHelpCommandHandler.kt
│   ├── MatrixQueryCommandHandler.kt
│   ├── MatrixReportCommandHandler.kt
│   └── SummaryFormatter.kt
├── audio/
│   ├── AlarmAudioCaptureCoordinator.kt
│   ├── AudioHintAnalyzer.kt
│   ├── AudioInputDeviceSelector.kt
│   ├── AudioRingBuffer.kt
│   ├── AudioTestRecorder.kt
│   └── WavWriter.kt
├── incident/
│   ├── IncidentRecord.kt
│   └── IncidentRepository.kt
├── matrix/
│   ├── HttpMatrixPublisher.kt
│   ├── MatrixConfig.kt
│   ├── MatrixPublisher.kt
│   ├── MatrixSettingsRepository.kt
│   ├── MatrixSyncClient.kt
│   └── MxcUploadResult.kt
├── sl400/
│   ├── AcousticMetrics.kt
│   ├── AcousticMetricsEngine.kt
│   ├── Sl400Decoder.kt
│   ├── Sl400Repository.kt
│   ├── Sl400Sample.kt
│   └── Sl400ViewModel.kt
└── ui/
    ├── cards/
    │   ├── AlarmAudioCard.kt
    │   ├── AlarmCard.kt
    │   ├── AudioTestCard.kt
    │   ├── CardShape.kt
    │   ├── ConnectionCard.kt
    │   ├── MatrixCard.kt
    │   ├── RecentSamplesCard.kt
    │   └── StatusCard.kt
    ├── screens/
    │   ├── HomeScreen.kt
    │   └── SettingsScreen.kt
    └── theme/
        ├── Color.kt
        ├── Theme.kt
        └── Type.kt

Additional UI image resources are expected under res/drawable, for example:

sl400_bg
sl400_meter
sl400_mock

Requirements

Hardware

Android phone or tablet with USB OTG support
Trotec SL400
appropriate USB OTG adapter / cable
optional USB microphone for audio capture

Software

Android Studio
Kotlin
Gradle
Android SDK compatible with Jetpack Compose

Network

Internet access for Matrix features
reachable Matrix homeserver over HTTPS

Permissions and Android Manifest Notes

At minimum, you should verify the following Android configuration.

Permissions

Common required permissions include:

android.permission.INTERNET
android.permission.RECORD_AUDIO
android.permission.POST_NOTIFICATIONS (Android 13+ if notifications are used)
android.permission.FOREGROUND_SERVICE
android.permission.FOREGROUND_SERVICE_CONNECTED_DEVICE

USB host feature

USB host support should be declared:

<uses-feature android:name="android.hardware.usb.host" />

Foreground service

The foreground service should be declared in the manifest and configured for connected devices.

Build and Run

1. Clone the repository

git clone <your-repository-url>

2. Open in Android Studio

Import the project and allow Gradle sync to complete.

3. Verify resources and manifest

Before running the app, make sure:

drawable resources are present
USB host feature is declared
foreground service is correctly configured
required permissions are present

4. Connect the SL400

attach the device through USB OTG
open the app
use auto-connect or scan devices manually
confirm USB permission

5. Configure Matrix

Open settings and enter:

homeserver base URL
access token
room ID

Then:

save settings
enable Matrix sending
send a test message

6. Configure alerts

Set:

threshold
hysteresis
interval
mode
metric
command room
target room
allowed senders

Save the alert configuration.

7. Enable audio capture if needed

grant microphone permission
start the alarm audio buffer from the UI or via Matrix command
verify the selected microphone input and sample rate

Troubleshooting

App crashes on USB connect

Likely causes include:

foreground service misconfiguration in the Android manifest
missing foreground service permission
Android version specific service restrictions
unsupported USB device / driver mismatch

Device is listed but cannot connect

Possible reasons:

missing USB permission
incompatible USB serial driver
device not exposing the expected serial port
cable or OTG issue

Matrix test messages do not arrive

Check the following:

homeserver URL uses https://
access token is valid
room ID is correct
Matrix account is joined to the target room
internet permission is present in the manifest

Matrix commands do not work

Verify:

commandRoomId is correct
sender is listed in allowedSenders, or the list is empty
Matrix sync is enabled through valid Matrix settings
the app's Matrix user is able to sync the command room

Alerts do not trigger

Verify:

alerts are enabled
threshold is reachable
the selected metric has enough window coverage
hysteresis is not preventing re-triggering
target room is valid
Matrix sending is enabled

No incident clip is generated

Verify:

microphone permission is granted
the alarm audio buffer is running
a usable input device is available
the clip file path exists after trigger

Clip upload fails

Check:

access token validity
Matrix media API availability on the homeserver
network connectivity
room permissions for sending audio messages

Known Limitations

the SL400 protocol is not fully documented
some tags are still unknown and only stored for debugging
no chart view or long-term measurement history visualization yet
recent samples are intentionally limited in the UI
alert state is local and intentionally simple
Matrix integration is lightweight and not a full SDK-based client
audio clips currently use basic metadata only
not all USB microphones and Android devices have been tested

Roadmap Ideas

Potential future improvements:

chart view for recent measurements
min / max / average statistics in the UI
JSON export of samples and incidents
better onboarding and connection diagnostics
clearer Matrix diagnostics screen
dedicated bot account setup guide
local Android notifications for alerts
richer alarm payloads and state reporting
support for additional SL400 protocol fields
optional automatic reconnect behavior
clip duration / waveform / upload status details
downloadable recordings directly from chat workflows

Disclaimer

This is an unofficial project based on reverse engineering and experimentation.

It is not an official Trotec application and is not affiliated with Trotec.

Protocol understanding may still evolve as more tags and behaviors are analyzed.

License

No license has been defined yet.

If you plan to publish this repository publicly, add an explicit license file such as:

MIT
Apache-2.0
GPL-3.0

Depending on how you want the project to be used.

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Trotec SL400 Reader for Android

Table of Contents

Overview

Current Features

USB / Device Features

Measurement Features

Acoustic Metrics Features

Alert Features

Audio / Incident Features

Matrix Features

UI Features

Project Status

Architecture Overview

How It Works

USB / Serial Communication

SL400 Protocol Decoding

Samples and Measurement Model

Acoustic Metrics Engine

Why coverage matters

Metric modes available for alerting

Alarm Evaluation

Supported alert settings

Alert modes

CROSSING_ONLY

PERIODIC_WHILE_ABOVE

Hysteresis behavior

Alert timing and audio hints

Incident Storage and Retention

Cleanup behavior

Audio Capture and Incident Clips

What it does

Audio sources

Audio controls currently available

Audio hint analysis

Output format

Concurrency guard

Matrix Integration

Sending

Sync

Remote control model

Daily reports

Background Service

User Interface

1. Home Screen

2. Settings Screen

3. Matrix / Remote Control View

Matrix Commands

Command behavior

Configuration

Matrix Settings

Notes

Alert Settings

Allowed Senders

Project Structure

Requirements

Hardware

Software

Network

Permissions and Android Manifest Notes

Permissions

USB host feature

Foreground service

Build and Run

1. Clone the repository

2. Open in Android Studio

3. Verify resources and manifest

4. Connect the SL400

5. Configure Matrix

6. Configure alerts

7. Enable audio capture if needed

Troubleshooting

App crashes on USB connect

Device is listed but cannot connect

Matrix test messages do not arrive

`CROSSING_ONLY`

`PERIODIC_WHILE_ABOVE`