Deemphasis Problem with External DAC?

Archived from groups: rec.audio.tech (More info?)

Hi, all,

I need some advice or suggestions/opinions about deemphasis switching
when using an external DAC with a CD player.

I recently purchased a used Musical Fidelity Tri-Vista 21 DAC. This is
my first experience using an outboard digital processor.

I am using an older (unmodified) Magnavox/Philips CDB-560 CD player as
a transport, connecting the coax digital output of the player directly
to the coax digital input of the DAC.

The combination seems to work very well, and the sound is an
improvement over the CD player's built in DAC and output stage.


I was fooling around yesterday, checking channel balance, frequency
response, square wave shape, etc. using my CBS CD-1 Test CD and
oscilloscope, and all looked good until I checked the deemphasis
frequency response.


The DAC is obviously not applying any deemphasis when required.


The Magnavox player itself applies deemphasis correctly, with the
proper response seen at its own analog outputs.


I can think of three reasons why the (expensive) Tri-Vista 21 DAC would
not properly deemphasize the test signal:

1) The processor is broken or not working properly... doubtful, because
the unit is otherwise working properly, and is not very old, and is in
excellent condition.

2) The processor is not designed to apply deemphasis, and does not
contain the required circuitry... Possible, but all of the other
Musical Fidelity DACs and CD/SACD players that I have been able to find
info on all contain deemphasis circuitry (and some of the cheaper ones
even have LED indicators on the front panel display).

3) The digital output signal from the Magnavox somehow does not
properly "activate" the deemphasis circuitry in the DAC.


I don't really understand the proper design of digital interfaces, but
I understand that the standard S/PDIF digital output signal is supposed
to contain a "flag" or "bit switch" that tells the processor whether to
apply deemphasis.

In the Magnavox player the deemphasis is applied as additional elements
in the feedback loop of an op-amp in the first filter stage (at the
player output), and is simply switched by an FET, directly controled by
a "DEEM" signal from the decoder chip.

The Magnavox player uses the Philips SAA7210 decoder, SAA7220 digital
filter, and the TDA1541 DAC chips.


Is it possible that the digital output from the Magnavox player does
not contain the proper "bit" to switch deemphasis in an external DAC?

If so, is there a way to "activate" the switch?

Or does anyone know if the Musical Fidelity Tri-Vista 21 actually
contains deemphasis circuitry?


I reallize that most CDs are not preemphasized, and I have no way to
check individual discs anyway, but I am definitely curious about this,
and interested in any wisdom or advice anyone can give!!

Thanks!!

Ken

Archived from groups: rec.audio.tech (More info?)

On 15 Jul 2005 12:24:49 -0700, spampup@aol.com wrote:

>I reallize that most CDs are not preemphasized, and I have no way to
>check individual discs anyway, but I am definitely curious about this,
>and interested in any wisdom or advice anyone can give!!

I would say that the MF, just like nearly all recent players or DAC's, don't
care about de-emphasis, which has been used on a very small number of CD's
years ago and is considered as obsolete by all manufacturers, even if still
part of the Red Book.

Archived from groups: rec.audio.tech (More info?)

"Norbert Hahn" <NorbertHahn@gmx.net> wrote in message
news:bh6gd1llvbhvo64hmlug05q39p8m0dksp1@4ax.com

> More recent DAC designs have the deemphasis implemented in
> software. A circuit diagram of the DAC should tell it.

Good point to bring out. A number of DAC chips have the
feature built in. But as a rule, the people who make the
equipment that those chips go into don't implement the
feature.

Archived from groups: rec.audio.tech (More info?)

>Have you ever heard of CDs actually using the pre/de-emphasis feature ?
>
>I think there is a very good reason why they chose not to bother
>implementing it !

I do have one or two which use this feature. The only one which comes
to mind is a sampler CD, which probably dates back to 1990 or so. I
don't think I've seen preemphasis used in any CDs made in the last few
years.

The sampler CD led to a somewhat confusing behavior... the CD player I
owned at the time would make a distinct "click" when changing from one
track to the next. Turned out that some tracks used preemphasis, some
did not, and the deemphasis logic was implemented via a discrete
low-pass analog filter (after the DAC per se) which was switched into
the circuit via a small relay on the CD player PCB.

--
Dave Platt <dplatt@radagast.org> AE6EO
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