multi-channel support and/or sp-dif

I've got a lot of multi-channel material in the form of DTS and Dolby Digital CDs and DVDs. I would like to listen to this stuff in my car - I currently have an outboard DTS decoder for my car. Ideally, the empeg would support Ogg Vorbis with 4 channels of output but since it doesn't look like Ogg will be available for an integer only platform I was thinking....


Would it be possible to get the empeg to play two mp3's simultaneously with the output of the first mp3 going to the front two outputs and the second mp3 going to the rear two ouputs? That way I could rip and re-mix the audio from multi-channel sources to two mp3 files and play them back that way. It might be nice to package the two mp3s into an uncompressed tar file to make house-keeping easier. Off the cuff, it sounds like this shouldn't be too hard, because of all the aggressive buffering that the player does now, it shouldn't be too hard to keep both mp3's in sync during playback.

Another alternative would be getting sp-dif out from the empeg and just storing the DTS tracks as wav files. I've read that there is something on the MkII board that makes getting digital audio out almost easy, but the details where not specified - is it simply a matter of attaching an rca jack to two contacts on the board or would it take more than that to get a standard sp-dif output working?

Would it be possible to get the empeg to play two mp3's simultaneously with the output of the first mp3 going to the front two outputs and the second mp3 going to the rear two ouputs?

This has been discussed before. Unfortunately, it's not possible because the player's DSP only accepts a stereo signal. The EQ and fader are all done in the DSP.

It's a standard off-the-shelf DSP made for car CD players, you see.

I am interested in I2C output for my own reasons. There's a good segment of the audiophile market that can post-process i2c and improve the audio significantly and produce an sp/dif output. So far, I don't have the full details for doing this but here is what I know to paraphrase hugo:

There is a 6 pin 2mm row of holes on the empeg board (not sure where) but it gives ground, 3.3v, i2c clock, i2c left, i2c right, i2c data1 and i2c data2 (front and rear). I'm mostly interested in the i2c format, but it's possible to use an sp/dif encoder (e.g. cirrus cs8405) and feed the sp/dif out an existing part of the harness, like the aux-in so you don't lose the docking convenience.

Hugo is the only person that knows the exact pin outs and location of that row of headers.

Comments?

Calvin

Just like you said, except replace I2C with I2S. I2C is a low-speed control bus (the empeg has one to talk to the DSP) and I2S is the high-speed synchronous audio data.

Hugo

oops.

Are the pinouts on the header labeled? Can you provide a picture with little arrows pointing out the pinouts? ;-) :-D ;-D

Calvin

Not labelled, no, they are: (pin 1 is indicated by a sloped corner on the bounding box)

1 - I2S clock (1.4112MHz)
2 - I2S word select (44.1kHz)
3 - Ground
4 - I2S data 1 (front L/R channel)
5 - I2S data 2 (rear L/R channel)

Next to this is the 2 pin I2S power header. Both pins are connected to the empeg 3.3v supply line.

The DSP can be programmed so that the digital outputs give other things - eg, ISTR there's a subwoofer filter/centre channel, which would allow you ot have the normal 4ch empeg output, plus add an external DAC to give you sub/centre out.

Sub is just something like a 5th order programmable lowpass, centre is just a mix of front L/R.

Hugo

centre is just a mix of front L/R.

Too bad it's not a difference of L/R, that would be more useful.

For simulating surround channels?

Calvin

No, for improving your car's front sound stage.

Not interested in surround, as I don't play any surround-encoded MP3s

In dolby terms, center is typically L+R and surround is L-R. You said it would be nice if it was a difference of channels..... so that implied you're looking to create some ambience effects in the cabin...

Calvin

I believe that the surround channel in dolby prologic is not L-R but rather anything that is in both L and R but is 90 degrees out of phase between the two channels.

Running regular 2-channel audio through dolby prologic often causes a "collapse" to the centre channel effect. By sending L-R to the centre and also sending the full L and R channels out you do get an "expanded" feeling of soundstage. Denon receivers with 5-channel and 7-channel stereo modes do exactly that and it sounds pretty good for casual listening seeming to "fill" the room pretty well.

Probably a little anal-retentive but I was just reading through my denon manual and I found that I was wrong they send the in-phase component to the centre channel. Sorry about that....

What's the part number for the DSP chip? Is a data sheet available on the web? I'm assuming it's some sort of off-the-shelf ASIC (is that a contradiction in terms or what?) rather than a general purpose DSP.

Speaking of I2C (not I2S), is there a convenient place to tack a couple wires for it? I have a pile of I2C temperature sensors left over from another project, and my car doesn't have a thermometer...

Thanks.

It's an SAA7705H and the spec sheet can be found in PDF format on the web.

Ah. I just glanced through the data sheet, and it looks like that's our DAC as well...