C24 Hardware Interface Manual
Version 0.3
2.8.6
Digital Audio Interface
The module’s digital audio interface is a serial Pulse Code Modulation (PCM) bus, which uses linear 2’s
compliment coding. The module is the PCM bus master, supplying the clock and sync signals to the
application.
The module’s digital interface is a 4 signal PCM bus, which includes a bit clock output signal for the bus
timing, a frame sync output signal for audio sampling timing, and serial data input and output signals.
The digital audio interface supports Voiceband audio – Intended for speech during voice calls and for
mono rings and alerts.
The PCM bus configuration is defined by the audio data format that is sounded through the digital audio
path, as described in the following table.
TABLE 12 – Digital Audio modes
Audio Mode
Frame-Sync
Sampling
Bit Clock
AT+CRTT tones
Voice
8 KHz
2048 kHz
Mono tones
8 KHz
2048 kHz
This digital voice audio format is used for speech during voice calls and for mono rings and alerts.
The PCM bus signal’s configuration for voiceband audio is:
•
PCM_CLK – 2048 KHz serial clock
•
PCM_FS – 8 KHz bit-wide frame-sync
•
PCM_DOUT – 13-bit linear audio data output
•
PCM_DIN – 13-bit linear audio data input
The analog audio is sampled at an 8 KHz rate and converted to linear 16-bit serial PCM audio data. The
serial data is transferred on the PCM bus in 16-bit word format,.
The 16-bit serial data is transferred in both directions after each sync signal’s falling edge. The sync
signal pulse duration is one clock period, after which the serial data is transferred in both directions for 16
consecutive clock periods.
Following the 16-bit data transfer, the serial input and output data signals inactivate until the next sync
pulse, which occurs every 125 uS (8 KHz). It is recommended the serial data signals will be High-Z during
the inactive period. The bus clock and sync output signals remain active all the time.
FIGURE 22 – Voiceband mode PCM bus coding format
PCM_FS
PCM_CLK
PCM_DOUT
PCM_DIN
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