From: gregr@tekig5.pen.tek.com (Gregory S Rogers)
Newsgroups: rec.video
Subject: New - FINAL PROOF about Sony TAE2000ESD Source Direct Mode
Date: 26 Jan 93 09:45:07 GMT
Organization: Tektronix, Inc., Beaverton,  OR.


The FINAL PROOF of how the TA-E2000 Source Direct Mode works is presented
in this article.  Read on, you WILL be convinced.  But first;

>When you connect up your TA-E2000ESD, set the unit to the "Source Direct"
>mode, over-drive the analog audio input, and then attempt to explain why
>the A/D overload LED comes on and the output goes into severe digital
>distortion on peaks.  The "processing circuitry" that the manual refers to
>as being by-passed are the DSP functions.  The analog signal is still
>going through the A/D->D/A converters.  (But I'd love to be proved wrong!)

>- David Altekruse, dna@ampex.com

Well I'm doing my best to prove David wrong :-).  Seriously, David and I
are discussing this off line as well.  I believe that either David has 
overloaded both the A/D and analog buffer amplifier in his test above, or
the relay in his unit which switches the signal perhaps isn't working.  In any
event I'm sure that an explaination will be found shortly.  I discuss the
possibility of overloading the analog buffer amplifier in a separate reply
in a different article.  Let me say here only that this condition can
be forced in testing but won't occur in real life use.  See the other article
for details.

This discussion may have already gone on too long for a limited audience so
unless some interesting new direction or discovery occurs this is it for me.
I apologize to anyone already bored but what follows might be interesting to
some readers in a general sense.  If anyone wants to continue the discussion
please contact me directly.

Proof that the A/D and D/A are NOT in the signal path in the source direct
mode:

1.  The block diagram in the manual.  I and another have discussed before.

2.  The overload proof that I proposed earlier.  This has since been
    verified by others. We still must explain David's results above.

3.  * NEW * The Frequency Response Proof. IMPOSSIBLE TO DISPUTE PROOF FOLLOWS. 


When the TA-E2000 is in the source direct OFF mode (path uses the A/D -> D/A)
there is a loss of high frequency information above 22 KHz. The frequency 
response is restored when the source direct mode is turned on.  How do I 
know this?  Obviously, because my Golden Ears can detect the loss of 30 KHz 
frequencies. (HA, HA just joking. We don't joke about such things in net.audio)

I measured the response of the preamp in both the source direct ON and
OFF modes.  Using pink noise and a real-time analyzer the frequency 
response is flat over the entire audio range including the octave centered 
at 31.5 KHz when the source direct is switched ON.  However, when the source 
direct is switched OFF, and the signal goes through the A/D and D/A, the
31.5 KHz frequency band drops by 10 dB!!  This change in response is caused
by the anti-aliasing and digital reconstruction filters that MUST be in the
path whenever the A/D and D/A is used.  This huge change in frequency response
ABSOLUTELY proves my point that the A/D and D/A is switched out in source 
direct ON mode.  If the A/D and D/A were in the circuit without the filters
the result would be garbage.  Since the filters are switched out, so too must
the A/D and D/A be switched out of the signal path.  

END OF ARGUMENT!       END OF DISCUSSION! 

Can you repeat this test without some type of frequency analyzer?  Yes, if 
you have a simple sine wave frequency generator and a voltmeter. Put in a 
low level (don't overload the preamp) sine wave at say 30 KHz and 
measure the output of the preamp with a common voltmeter (an
Oscilloscope is better if you have one).  Do this with and without the source
direct switch on and you should see a large difference in output.

Well that's the end.  If anyone wants to discuss other aspects of the 
TA-E2000 I'll be happy to join in.

Greg Rogers

Newsgroups: rec.video