I've recently used a DIT4096 to try and output SPDIF/TOSLINK from a device which has a DSP with 8 bit output only. Needless to say, the DIT4096 will not accept 8 bit input whilst 16 bit if fine.
I've tried to find a circuit to convert 8 bit PCM to 16 bit PCM, without success.......
I did however find this:
http://www.ti.com/general/docs/lit/getliterat … 93&fileType=pdf
Any thoughts?
Which DSP and what kind of output format it has?
wrote:Which DSP and what kind of output format it has?
It's a TI TMS320C17 which has an 8 bit serial output.
wrote:wrote:Which DSP and what kind of output format it has?
It's a TI TMS320C17 which has an 8 bit serial output.
But how many bits per frame, or is it variable? What DAC chip it is connected to? Is it even using standard audio sampling rates?
The more info you give the more info I can give. So basically, what are you trying to do, build a SPDIF output to what exactly?
The DAC is a TCM128T16N, which is mainly used in telephony applications.
It sends/receives u-law encoded PCM. So I'm trying to add SPDIF to a device with 8 bit u-law encoded PCM.
wrote:The DAC is a TCM128T16N, which is mainly used in telephony applications.
It sends/receives u-law encoded PCM. So I'm trying to add SPDIF to a device with 8 bit u-law encoded PCM.
Can't find any reference or datasheet to that chip. Are TCM29 chips close enough?
You haven't mention what device it is you are modifying and why, is this a secret?
Well you are faced with several different things that either are compatible or not and you need to figure them out.
Even if there is 8-bit audio on the bus, it is possible there are more than 8 bits per frame so if there is say 16 bits per frame or whatever is compatible with DIT4096 then you can connect them. So basically, you need to figure out what is the bus format used between TMS and DAC in your device and if it matches the DIT4096 requirements, regarding also the aspect that there is one mono audio channel and the DIT4096 can support mono mode somehow. The sampling rate just needs to be something fixed, and you need to arrange a master clock for DIT4096. If you get all these right, then the DIT4096 can output your data through SPDIF.
Then you have two other problems.
If the sampling rate is not a standard audio sampling rate, what device would accept SPDIF signal with non-standard sampling rate? All devices I know just eat standard 32/44/48/88/96/176/192 kHz sampling rates.
And even if it does work, it would sound distorted, if the audio on the bus is u-law compressed instead of linear PCM.
So it is possible that you need something in between the TMS320 bus and DIT4096 to make them compatible with each other (like a CPLD, FPGA, another TMS320 or other DSP, microcontroller or whatever that can convert data to more reasonable format, etc).
8-bit u-law is actually a sort of compression algorithm, so your raw samples are more than 8-bit linear PCM (probably 14-bit).
What you probably need to do is u-law decode it (which I believe can be done with just a lookup table), and while doing that, you can promote it to 16-bit linear PCM.
wrote:8-bit u-law is actually a sort of compression algorithm, so your raw samples are more than 8-bit linear PCM (probably 14-bit).
What you probably need to do is u-law decode it (which I believe can be done with just a lookup table), and while doing that, you can promote it to 16-bit linear PCM.
Yes, I've done some more reading now, however there must be a chip that is designed to do this for telephony applications. However, I simply cannot locate one, it appears that the telephony worlds 8 bit sound and the 16 bit+ pc world have always kept their distance.
This will do the job:
http://www.electronics-eetimes.com/en/mp3-enc … ws_id=222906936#
http://www.lpsystems.eu/products/24-embedded-mp3?showall=
https://www.sparkfun.com/products/11684#comme … e395f171d8b4568
however I don't want to have to code........