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Old 28th December 2003, 10:03 AM   #1
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Default Buffer choice for IIS Direct

While searching in my drawers for the 74CH125 specified in Elso's schematic I found out that I have none.

Since it seems to me a ordinary buffer/line driver I started looking for equivalents.

I have a lot of boards to salvage that mount the 244 octal buffer in various flavours (HC, HCT, F, AC...), as well as some 74F125 which is twice as fast as the HC.

Can I use this or the "F" series carries some issues that make it unsuitable?

Among these logic families, which ones can work best?

Attached is the schematic I'm referring to (posted by Elso Kwak)

Cheers

Andrea.

Edit: the 5V can be taken from the player(maybe with some LC filtering or I'd better use a separate PS?)
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Old 29th December 2003, 02:58 PM   #2
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Hi

Why adding the buffers ?

Suggestion: Place the master clock close to te DAC ad feed it backwards.......

Ciao
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Old 29th December 2003, 03:16 PM   #3
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Quote:
Originally posted by Guido Tent
Hi

Why adding the buffers ?

Suggestion: Place the master clock close to te DAC ad feed it backwards.......

Ciao
Hello Guido,
I think Elso added the buffers to drive a coax cable for each data line, since the DAC itself is placed outside the CD player housing for RFI reasons(I suppose).

Initially I was thinking to omit this circuit and place my DAC inside the player (adding a supply for it), but I experienced serious RFI problems only placing my external TDA1543 DAC on the top of the player (Philips CD931) so now I'm quite doubtful about it.

For the clock, what do you mean? I think I'll replace the player's one but AFAIK no other clocks are needed using IIS direct..
can you elaborate a little?

Cheers

Andrea

Edit: partial answer to my question.. using a TTL buffer followed by a emitter follower greatly degrades the noise margin of the circuit, so I think I must use a HC one.
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Old 29th December 2003, 03:57 PM   #4
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Quote:
Originally posted by Andypairo


Hello Guido,
I think Elso added the buffers to drive a coax cable for each data line, since the DAC itself is placed outside the CD player housing for RFI reasons(I suppose).

Initially I was thinking to omit this circuit and place my DAC inside the player (adding a supply for it), but I experienced serious RFI problems only placing my external TDA1543 DAC on the top of the player (Philips CD931) so now I'm quite doubtful about it.

For the clock, what do you mean? I think I'll replace the player's one but AFAIK no other clocks are needed using IIS direct..
can you elaborate a little?

Cheers

Andrea

Edit: partial answer to my question.. using a TTL buffer followed by a emitter follower greatly degrades the noise margin of the circuit, so I think I must use a HC one.
Andrea

If you use the buffer, then why the E-follower ?

Clock should be as close as possible to the DAC, where the party is ! Feed it back to help generating I2S

RFI: Weird, shouldn't be a problem to combine a drive and a DAC chip.....

Ciao
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Old 29th December 2003, 04:39 PM   #5
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Quote:
Originally posted by Guido Tent


Andrea

If you use the buffer, then why the E-follower ?

Clock should be as close as possible to the DAC, where the party is ! Feed it back to help generating I2S

RFI: Weird, shouldn't be a problem to combine a drive and a DAC chip.....

Ciao
I think the e-follower is to drive the 50 Ohm-terminated cable (surely Elso can explain us his choice much better than me), if the DAC is kept internal maybe the buffer is also unnecessary (unless you parallel too many chips)..

So the clock should feed the DAC (like in DDDAC ?) and then be brought back to the transport?

About RFI I remember that the SPDIF output of my player wans't considered that well... maybe it's just a problem of that interface...

Cheers

Andrea
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Old 29th December 2003, 04:42 PM   #6
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Cool I2S Direct

Hi Andrea and Guido,
What, do I have to explain this very simple circuit to an EE? Please read for CDP: CD player component and DAC is DAC component. These are two cases connected with 60 cm 50 Ohm coax cables.
As Andrea indicated the Masterclock is not needed or used in the NON-OS DAC. So why bring it back? I explained this earlier to Lars Clausen. Is the NON-OS concept so difficult to digest? In your words "there is no party for the Masterclock at the NON-OS DAC".
Then the circuit is taken directly from Horowitz page 612 fig. 9.42 in my second edition. Here is also explained why direct logic drive of long coax cables is not feasible.
Now for the buffer: why it is there? No, I don't have to explain that....[hint: figure what happens if you short the transistor...] Oh no, leave the buffer away but don't email me helping solving the problem...
Andrea I only had at hand 74HC125 in DIL and did not want to fiddle with MC74VHC125 in SMD. Maybe VHC logic works better here, but I have not yet tested that. AC logic should be avoided as the black pest and F logic is obsolete. HCT or VHCT will probably also work.
Ciao
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Old 29th December 2003, 04:42 PM   #7
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The E followers are here to compensate the 0.5 gain of the transmission line, I think.

Guido: the TDA154x dacs have no clock input (except the BCLK from IIS) and this way, no clock output.
I don't think that placing the clock near this specific DAC will help
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Old 29th December 2003, 04:44 PM   #8
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I went here too late
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Old 29th December 2003, 04:47 PM   #9
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Cool I2S Direct

Quote:
Originally posted by Bricolo
The E followers are here to compensate the 0.5 gain of the transmission line, I think.

Hi Bricolo,
Sorry wrong answer!
There are no 50 Ohm resistors at the transmittor end of the cable. In fact the overall gain is close to one but <1.
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Old 29th December 2003, 05:00 PM   #10
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dear all,

EVERY DAC needs a clock to convert the data to something analog, regardless of the level of oversampling. Is DA conversion so difficult to digest ? Needless to say that a 1541 has a clock input.

If you use asynchronuous clocking, you can have a free running clock close to the DAC (but I am still not convinced of this principle)

Buffers: The example is too complex. I suggest an E follower, current source loaded with a 47 ohm series resistor to the output, to make it a characteristic driver impedance

Elso, others, I'd suggest to use true transmission line with proper driving / terminating. The jitter (for example due to non characteristic system) in the I2S signals definitely ripples through the DAC, wasting the sound (regardless of the clock used there).

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