Using digital out from Discman

[kevinkr]

Originally posted by DocLorren
I would say if there is not a big choice in affordable DIY DACs that accept I2S data then there's no reason to go for an external DAC. It may be better to find a good quality I2S->DAC for internal use. I know about the parallel TDA1543 trick but feel this is a kind of silly compromise. Is it better at all than the standard Discman DAC? Has time stood still since the development of the TDA1543 (1991 right?) and sonically superior chips not been developed in the last decade? And what exactly is the reason behind using RS232 in case of two housings? Timing problems or other? More questions the deeper we go into this topic. Thanks in advance,

Wolfson WM8740 (voltage out) or TI PCM1794/1798 (current output) - all in small SSOP packages. (A bit harder to work with than SOIC). I'm using the much cheaper PCM1798 in some Twisted Pear Audio COD dac boards I built a couple of weeks ago - quite pleased with the sound.

[DocLorren]

Thanks Kevin,
I will sure check out these chips. Is parallelling still beneficial/necessary?

[kevinkr]

I am actually running a pair of PCM1798 in mono mode with the L and R outputs connected in parallel. (If you do this remember to reverse the connections on one channel as TI designed them to swap when mono mode is invoked.) Each time you double the number of dacs in parallel you get a 3dB improvement in noise performance and linearity, but you rapidly reach the point of diminishing returns - doing this makes the most sense if you are using passive I/V conversion as it results in higher output voltages for a given I/V resistor, and therefore a lower source impedance for a given output level.. I prefer some sort of active I/V conversion in most cases or very small resistors for I/V conversion with a high quality gain stage to make up the lost gain.

[amc184]

I know about the parallel TDA1543 trick but feel this is a kind of silly compromise. Is it better at all than the standard Discman DAC? Has time stood still since the development of the TDA1543 (1991 right?) and sonically superior chips not been developed in the last decade?

There are a few of reasons why I particularly mentioned the paralleled TDA1543:
- You can get TDA1543 chips cheaply (~US$2.00 each).
- These chips parallel very well. Almost every DAC chip will benefit somewhat from being paralleled, but the TDA1543 benefits a huge amount, and make for a good DAC. A single TDA1543 is no much good in my opinion.
- There are not the many external components to a TDA1543, and the chip comes in DIP package, so it is easy to build on of these using prototype board. So again, it is cheap.

And have DAC chips increased in quality since 1991? Of course. Better chips have come along, such as the mentioned PCM1794 and WM8740. However, a lot of the 'progress' has been focused on making the DACs cheaper, not better (see bitstream). Almost any DAC is going to be better than the walkman's. I suggested the TDA1543 as it is cheap and easy, sounds reasonably good, and will make an excellent first project.

[amc184]

And what exactly is the reason behind using RS232 in case of two housings?

I2S is a format intended to be used between audio ICs within the same device. It shouldn't really be used over a distance of much more than 10". If you want to go further than this, you should you a transmitter and receiver. Something like a MAX234 (TX) and MAX208 (RX) pair would be fine. These boost the voltage of the signal, and allow it to carry over a distance. For short runs (<1m), you might be able to get away without it, but you are asking for trouble.

[jkeny]

Hey DocLorren,
Good project - I have an old discman and I have a number of TDA1543 DACs all of which is just waiting for a project. Could I get the Service Manual from you, please?

There's also a TDA1387 DAC which is a TDA1545 but will accept I2S

[kevinkr]

Originally posted by amc184


I2S is a format intended to be used between audio ICs within the same device. It shouldn't really be used over a distance of much more than 10". If you want to go further than this, you should you a transmitter and receiver. Something like a MAX234 (TX) and MAX208 (RX) pair would be fine. These boost the voltage of the signal, and allow it to carry over a distance. For short runs (<1m), you might be able to get away without it, but you are asking for trouble.

RS-232 will not be fast enough.. RS422 should be ok with the I2S for clocks of less than 10MHz, but may be getting marginal above this, RS-485 is faster still.. Note that you could just use the DB9 connectors without any drivers with coax, but the distances with I2S need to be very short. I had pretty serious malfunctions in my dac when I had I2S lines longer than 15cm or so..

Incidentally the MAXIM parts cited in the above post are far too slow, topping out at 120kbits per second. You need something 100X faster to do this job depending on mclk (sclk) speed.

[DocLorren]

Ok guys, thanks for your input so far, I think I know which direction to take from here (internal DAC, starting with parallelled TDA1543s). Projected costs for a (hopefully) good sounding micro CD-player:
Discman: 15 euros on Ebay (adapter included, to be transformed into internal PSU)
Internal I2S DAC: 30 euros (less if PSU from Discman can be used)
Hammond casing: 20 euros
Connectors, switches etc: 20 euros
Mistakes (definition will follow after build): 15 euros
Total: 100 euros
Funfactor: priceless...again

Just a few more questions (especially directed at Anton now that you have read the SM): can the RAM controller be bypassed without trouble? What IF I want to experiment with regular external DACs (optical or coaxial coupled). What would be the best/simplest solution to convert an I2S signal into SPDIF? I do not think I will follow this route but it might be interesting anyway.

@JKeny:
Which model do you have? My SM is covering the D-E44x line.
Drop me an email.

[amc184]

The internal DAC sounds like a great idea. You definitely should bypass the ram controller.

can the RAM controller be bypassed without trouble? What IF I want to experiment with regular external DACs (optical or coaxial coupled). What would be the best/simplest solution to convert an I2S signal into SPDIF? I do not think I will follow this route but it might be interesting anyway.

I cannot guarantee that the RAM controller would degrade the sound, but it would certainly not improve it.

There is no easy way to get a SPDIF out, it will require an extra IC, something like a CS8406.

Internal I2S DAC: 30 euros (less if PSU from Discman can be used)

No, you'll need a seperate PSU for the TDA1543 stack, something providing regulated DC of 5 to 8VDC. I would also knock up a seperate PSU for the walkman, of 3.3V. The walkman PSU could also supply a CS8406 if desired.

[amc184]

Hang on, on further inspection of the service manual, the transport DC supply should be 4.5VDC to the charge port.

Anton