digital audio paths (Was: Re: Experimental technique for testing call audio quality? )

Helge Hafting helge.hafting at
Mon Jan 25 13:03:26 CET 2010

Stefano Cavallari wrote:
> On Friday 22 January 2010 20:13:58 Timo Juhani Lindfors wrote:
>> Helge Hafting <helge.hafting at> writes:
>>> the same quality as this, by tweaking volume settings. Calls are digital
>> The audio data is fed to the GSM chip in analog form however.
>>> If the FR sound quality is too bad, consider a BT headset. Sound quality
>>> should then depend on the BT headset only. The FR's problems with buzz,
>>> bad bass, and possibly other analog issues shouldn't matter at all.
>> Even in this case the audio is fed in analog form to the GSM chip.
> Why on hell GSM chips are still done this way?

Good question. "Don't touch what works" maybe?

> I hope whatever next open phone comes around it's totally digital.
> Audio lines maybe were a good idea for phones with few cpu power available, 
> but now it's only a waste of PCB space I think.

CPU isn't much of an issue for playing (uncompressed) digital sound.
A small buffer and a DAC is all you need. I guess they put that stuff 
into the GSM chip, to avoid an extra audio chip. The most common use for 
GSM audio is to send it to the speaker, so the design makes sense.

> The hardware becomes more flexible and maybe you can do some DSP to enhance the 
> voice. Or implement an answering machine inside the phone. 
> Do anyone know about recent GSM chips? I'm still dreaming a up-to-date open 
> phone :)
> Is there a standard to do voice-audio-codec over multiplexed AT channel?

There is a standard for digital voice over an AT channel. The modems we 
used before ADSL took over had this - and so you could implement an 
answering machine on your PC. Or even menu systems with "press 1 to 
leave a message, press 2 to ..." It wasn't multiplexed though.

The AT command set is documented and can probably be found on the net. 
So you could try some commands with a terminal emulator program on the 
FR. GSM supports data calls, maybe you can
get digital voice too.

Helge Hafting

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