Latency Measurement¶

Measurement used to characterize the device latency of the PROP audio path. The measured path starts when the dongle firmware recognizes a marker in a USB speaker packet and ends when the corresponding marker appears at the headset codec analog output.

Measurement Principle¶

USB host plays a 48 kHz, stereo, signed 16-bit PCM WAV containing full-scale marker samples.

On the dongle, path_dongle.c scans each PROP_SPK_PACKET_BYTES speaker chunk after it has been read from TinyUSB's EP OUT FIFO. When the marker pattern is found, the dongle toggles GPIO P0.02. This GPIO edge is the measurement start time.

The same marker samples are then transmitted to the headset over PROP and played through the codec. The resulting analog pulse at the codec output is the measurement stop time. The oscilloscope measures the time delta between the dongle GPIO edge and the analog output pulse.

Marker Format¶

Marker stimulus:

Sample rate: 48 kHz
Channels:    2
Format:      signed 16-bit little-endian PCM
File:        python/latency_marker_48k_stereo.wav
Generator:   python/generate_latency_marker_wav.py

Marker sample sequence:

+32767, +32767
-32768, -32768
+32767, +32767
-32768, -32768

The marker interval is aligned to the 1 ms speaker packet boundary. This keeps the marker pattern inside a packet-sized chunk and allows deterministic marker detection in path_dongle.c.

For the recorded measurement, the WAV was played from the USB host through a direct ALSA hardware PCM device. This avoids host-side mixer gain, resampling, or effects that could modify the full-scale marker samples.

Measured Boundary¶

Measured interval:

dongle P0.02 marker GPIO edge -> headset codec analog output pulse

The measured interval includes:

dongle processing after the speaker packet has been read from TinyUSB EP OUT
PROP TX enqueue and radio transfer
headset PROP RX handling
headset speaker FIFO buffering
headset I2S scheduling and DMA
codec output latency

Probe Points¶

Scope channel	Signal	Function
CH3	Dongle P0.02	Marker detection edge, measurement start
CH2	Headset codec analog output	Reconstructed marker pulse, measurement stop

P0.02 is used for the audio latency marker.

Result¶

Latency scope capture

Capture file:

docs/RigolDS9.png

Oscilloscope measurement:

Rigol Delta 3->2
CH3 marker edge to CH2 analog pulse
100 marker events

Recorded statistics:

Statistic	Latency
Average	7.8529 ms
Minimum	7.5456 ms
Maximum	8.2056 ms
Standard deviation	127.26 us

Limitations¶

The measured interval excludes:

application/media-player latency on the USB host
host OS audio stack latency
USB host scheduling before the packet reaches dongle firmware
time spent queued in TinyUSB's EP OUT FIFO before path_dongle.c drains it

This is therefore not an end-to-end PC playback latency measurement. It is a device-path latency measurement referenced to the point where dongle firmware has already consumed the USB speaker packet containing the marker.