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Old 10th August 2005, 07:39 AM   #1
diyAudio Member
Join Date: Aug 2005
Default A different problem: analog phone to digital interfacing

Hi, this is perhaps somewhat of a strange problem for this forum, but I'm hoping someone can help me out. I am trying to put together a portable that has a cell phone engine and decent quality audio based on wolfson codecs. I'll get to the HiFi audio issues later, but my problem is connecting the outputs of a GSM cellular module to a codec. Essentially I want to get everything in the system into I2S format and use a mixer to switch between audio sources. It would be easier if the GSM module had digital output, but it does not, and I'm not sure of the best way to connect the cell module to the codec. First, specs on the audio on the cell module:

The output is intended to be an earphone:

Line Coupling: DC
Line Type: Bridged
Speaker Impedance: 16 O +/- 5% @ 1 kHz
Minimum Load Impedance: 15 O
Signal Bandwidth: 150 - 8000 Hz @ -3 dB
Maximum Differential Power Output: 850 mV
Rated output power: 10 mW
Maximum Power Output: 30 mW

The input is intended to be a microphone:

Line Coupling: AC (100 nF cond)
Line Type: Balanced
Differential Input Resistance: 25kO
Line Nominal Sensitivity: 50 mV rms
Max Input Voltage: 360 mV rms
Microphone Nominal Sensitivity/Analog Gain Suggested: -45 dB Vrms/Pa / + 24 dB

As mentioned earlier, the quest is to get these signals in and out of I2S format. Wolfson has a lot of flexibility for this, and my initial plan was to use this dual codec, which is PCM on one side and I2S on the other:


This codec is meant to be connected to a digital PCM source (not analog), but the codec has a variety of configurable inputs including differential. Since this is a mono source I thought maybe a mono codec like this would be better:


I'm one of those software people with dangerous amount of hardware knowledge hehe. What I understand so far is that the cell module uses balanced (not single ended) signals for input and output, and fortunately both of these codecs can accomodate that. However, the codec inputs are AC coupled while the module outputs are DC coupled, and of course the opposite is true also in the other direction. So these are my two main questions:

1) Should I be trying to convert the module speaker (earphone) level signal to line level for input to the codec (and what circuit would do this?), or should I be connecting to microphone inputs on the codec? Same idea in the other direction from codec out (it has line level and mono speaker outputs)

2) How can I convert the DC coupled output of the cell module to AC coupling for input to the codec, and vice versa for DC coupled output from the codec to AC coupled input on the module?

Any suggestions would be greatly appreciated. The idea here is true intregration of features: cellular voice (GSM) and data (GPRS), speech synthesis (hardware), and maybe not HiFi by your standards but decent quality audio. Wolfson codecs are ideal for this I think, especially in the power consumption and size factors. But I'm struggling with this phone interface. Any ideas?

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