Why Philips CD 723 is a bad base for modding

[OliverD]

Recently I aquired a low-cost Philips CD 723 player. I paid around 40 Euros including shipping.

It is a very low cost device, but it can read CD-RW without problems. My initial plan was to refit a new power supply, clock and internal DAC which would be fed directly by the player's I2S. Going non-OS, I figured it would be a very good sound for little money project.

Here's why it won't work as intended:

The CD 723 uses a one-chip servo/decoder/digital filter solution which belongs to the CD7 family from Philips. This chip features an internal digital filter, oversampling to either 2fs or 4fs. You can chose between those two filters by software, but you cannot disable them. So no non-OS unless you use the chip's biphase-mark output.

The other problem is the digital filter in the SAA7378 is mediocre at best. The internal resolution is not sufficient at all, stop-band attenuation is very poor, and a -0.5dB downscaling is applied on all input signals to prevent overflows. The chip doesn't even dither before this downscaling. Thus, you will never get accurate samples from the CD out the I2S of the chip!

So, don't buy CD 723, although it is recommended in several threads in this forum!

[tschrama]

True, but if you had search the forum, you would jave known that before you bought it.

Still, I do recommend the CD723, because it is cheap, it's a simple design, it's easy to modify.

Your problem (and only 1 problem) is that your limmit your self to the I2S output. Don't use it. But by now, you know that too. Use the S/PDIF. If you worry about jitter: use a low jitter clock near you favorite DAC and slave it through to the player... that's the ultimate solution, for any player.

Good luck,

CD723, still highly recommended!

[OliverD]

True, but if you had search the forum, you would jave known that before you bought it.

Sometimes I do things without asking first

Use the S/PDIF. If you worry about jitter: use a low jitter clock near you favorite DAC and slave it through to the player... that's the ultimate solution, for any player.

The -0.5dB scaling still applies, even when you use the S/PDIF. It is done in a very mediocre way, without dither and - of course - asymmetric around digital zero. It affects sound quality and prevents you from getting the exact sample values (as on the CD) out of the decoder chip.

Thus, I really cannot recommend CD 723 and similar players using the SAA7378.

[aparatusonitus]

[i]

CD723, still highly recommended! [/B]

Particularly if you use dig. output from SAA7378 to feed 4xTDA1543A!!!

[OliverD]

Particularly if you use dig. output from SAA7378 to feed 4xTDA1543A!!!

Sure sounds better than most CD thingies you can buy, but you'd be closer to audiophile heaven if you did that mod to a different player which doesn't corrupt the digital data.

That's all my post was about.

[00940]

This website (it's not mine) explains how to hack the controller to get 1fs dataflow. http://www.ifrance.com/robinetron/cd723/hack/hack.html

Sorry it's in french.

[tschrama]

The -0.5dB scaling still applies, even when you use the S/PDIF. It is done in a very mediocre way, without dither and - of course - asymmetric around digital zero.

If that's true, it's new for me and indeed something to consider.

Could you explain it a little more ellaborate.. howcome that the S/PDIF is affected? assymetric attenuation around zero? What's that?


Regards,
Thijs

[OliverD]

This website (it's not mine) explains how to hack the controller to get 1fs dataflow.

Although I can't read french, I understood he is using an Atmel controller to interfere with the communication between the SAA7378 and the player's controller to alter the output format setting.

In the SAA7378 datasheet from 1995, page 15, there is a table which shows six modes the DAC output interface can work in. All of them are either 4fs or 2fs. However, in his program, is he using none of these documented modes, but rather an "invalid" combination of bits.

Now there are two possibilities:

1. This guy is really clever and remembered the SAA7378 is basically an advanced SAA7345 and a TDA1301T on one die, and the "old" 1fs modes from the SAA7345 still work in the SAA7378, although they don't appear in the datasheet.

2. He took the output format codes from a website or from a different data sheet (like the one for SAA7345), assumed they are the same for the newer chip and tried them. They don't work and the chip remains in its default setting, which is 4fs.

From looking at his website, he seems to be very knowledgeable, so I hope it's the first possibility.

If that's true, it's new for me and indeed something to consider.
Could you explain it a little more ellaborate.. howcome that the S/PDIF is affected? assymetric attenuation around zero? What's that?

It seems the downscaling multiplier is before the biphase-mark output in the SAA7378 block diagram.

This guy really had a close look at the issue and he explains it much better than I could.

It looks like my warning finally can be refuted (which is in my best interest, after all I own one of those machines), but some issues need to be sorted out before I agree:

- Can we really circumvent the digital filter section of the SAA7378 for exact bit-for-bit accuracy, by setting the output to this undocumented, "illegal" mode?

- Is there a way of disabling the mediocre -0.5db downscaling?

[jkeny]

Excellent find - I knew somebody would program a microcontroller for the CD400 (CD723) someday - I and others looked at this about a year ago and realised the SA7378 could be changed to no oversampling mode but only by sending signals to it from a microcontroller.

Looks like this is what this guy did it - ther is a lot of activity in France on this player in the site http://forums.delphiforums.com/HAUTE_FIDELITE/start
but mostly in French.

Unfortunately my French is not up to par

PeuFeu has an interest and is himself a French speaker i believe maybe he will let us know the translation.

In the meantime I sent robinetron an email to get details in english - will post them here.

Now to clear up the final

[carlosfm]

Originally posted by OliverD
The -0.5dB scaling still applies, even when you use the S/PDIF.

If that's true, that player used as a transport with an HDCD external DAC will never play in HDCD mode.