Chapter 2: Hardware Interface Description
January 1, 2006 Module Hardware Description 35
The digital audio interface supports 4 types of audio data formats, which define the PCM bus
configuration and data rates:
• Voice band audio - Intended for speech during voice calls and for mono rings and alerts.
• Stereo audio - Includes 3 audio formats that support high quality stereo ring tones and alerts.
The PCM bus configuration is defined by the audio data format that is sounded through the digital
audio path, as described in Table 2-10.
Voiceband Audio
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 - 512 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 13-bit serial PCM audio
data. The serial data is transferred on the PCM bus in 16-bit word format, which includes 13
sampled data bits, and 3 added zero value bits.
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 µS (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.
Important: In digital audio mode the input and output gains cannot be controlled by AT
commands.
Table 2-10:
Digital Audio modes
Audio Mode
Frame Sync
Sampling
Bit Clock AT+CRTT Tones
Voice 8 kHz 520 kHz
Mono tones 8 kHz 520 kHz 6 - 9 (7 default)
Stereo low tones 20.05 kHz 705 kHz 12 - 38, 40 - 46
Stereo mid tones 32 kHz 1024 kHz 48 - 49
Stereo high tones 44.1 kHz 1.4 MHz 10, 11, 39, 47
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