PCM Protocol Support

Audio

Pulse Code Modulation

What is PCM?

PCM (Pulse Code Modulation) in the context of digital audio interfaces refers to the synchronous serial bus commonly used for telephony, Bluetooth audio, and voice processing applications. A PCM interface uses four signals: CLK (bit clock), SYNC (frame sync), DIN (data in), and DOUT (data out). Unlike I2S which is designed for stereo hi-fi audio, the PCM interface is optimized for voice-bandwidth audio at 8 kHz or 16 kHz sample rates with 8-bit or 16-bit resolution. PCM interfaces are widely found connecting Bluetooth modules, cellular modems, and voice codecs to baseband processors. Engineers need to verify frame sync timing, clock accuracy, data alignment, and slot assignment when debugging audio path issues in these systems.

PCM Quick Reference

type Serial, synchronous
signals CLK, SYNC, DIN, DOUT
max Speed Up to 8 MHz
voltage Range 1.8V – 3.3V
features Telephony audio interface

Acute Instruments Supporting PCM

Recommended Solutions

Recommended for Decode

TL4234B

TL4234B

All Supporting Products

Protocol Decode
Hardware Trigger
Protocol Exerciser

Supported Product Families

Protocol Decode
Hardware Trigger
LA4000 Series
LA4000 Series
TravelLogic Series
TravelLogic Series

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How to Analyze PCM with Acute Instruments

1

Connect your Acute logic analyzer to the PCM CLK, SYNC, DIN, and DOUT signals at the codec or Bluetooth module.

2

Attach a ground lead to the target board's ground reference.

3

In the Acute software, select the PCM protocol decoder and assign each signal to the correct input channel.

4

Configure the sample rate, bit depth, frame sync polarity, and slot count to match your audio configuration.

5

Capture traffic and view decoded audio samples for both input and output directions, verifying timing and data alignment.

Frequently Asked Questions

What sample rate do I need for PCM interface analysis?
PCM bit clocks typically range from 256 kHz (for 8 kHz mono, 8-bit) to 4 MHz (for 16 kHz stereo, 16-bit with multiple slots). Sample at a minimum of 4x the bit clock frequency. For a 2.048 MHz PCM clock, use at least 10 MHz sampling. These rates are well within the capability of all Acute logic analyzers.
Why does my PCM audio sound distorted or have channel swapping?
Distortion and channel issues on PCM buses are commonly caused by frame sync polarity mismatch, incorrect data delay (first bit position relative to sync edge), or clock edge configuration (data sampled on rising vs. falling edge). Use the protocol analyzer to capture and verify that the SYNC signal timing, data bit alignment, and clock polarity match between the master and slave devices.
How many channels do I need for PCM analysis?
A full PCM interface requires 4 channels: CLK, SYNC, DIN, and DOUT. If you only need to monitor one direction (e.g., codec output to Bluetooth module), 3 channels (CLK, SYNC, and one data line) are sufficient. Some designs share a single bidirectional data line, reducing the total to 3 channels.

Related Protocols

I2S TDM

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