From: gregr@tekig5.pen.tek.com (Gregory S Rogers)
Newsgroups: rec.video
Subject: Re: Sony TA-E2000ESD Source Direct
Keywords: Dolby prologic
Date: 26 Jan 93 08:36:05 GMT
Organization: Tektronix, Inc., Beaverton,  OR.


Since several people are trying to verify my explaination of how the TA-E2000
works in source direct mode, I will add a few more comments about the 
demonstration that I suggested.  See comments below.  Also in a reply to
another posting I will provide yet another way to prove that the A/D & D/A 
are NOT in the signal path in the source direct mode.

In an earlier posting I said the following:

>                                       In the bypass mode the analog signals
>go through the input level control then directly to the master volume control.
>They do NOT get converted through the A/D->D/A in this path as you suggest.
>This can be seen in the Sony block diagram on page 52 of the manual.  Its the
>line from the center tap of the input level control to the switch connected 
>to the top of the motorized ganged master volume control.  
>
>This topology can be simply demonstrated.  Play a CD, using the ANALOG output
>of the CD player into the (obviously) ANALOG input of the TA2000.  Now turn the
>volume control down to a fairly low level.  (I don't want to hurt your
>speakers.)  Now WITHOUT the bypass switch on, turn the input level control, NOT
>the VOLUME control, all the way to max. During louder portions of the CD the
>OVERload indicator on the TA2000 will light up and the sound will be awful.
>Clicks, pops, distortion etc as the A/D convertor is overloaded.  Now press
>the bypass switch and notice the sound is now fine.  The OVERload indicator
>will still light because the signal still goes to the A/D but since the
>signal from the TA2000 bypasses the A/D it sounds fine. The A/D and obviously
>D/A are bypassed by the switch.  This is the desired design topology. (When 
>using digital input signals the signals are routed through the D/A in the 
>bypass mode, which of course they must to get an analog output.)

Steve Kanefsky replied:

>>Today, I finaly got a chance to do it and I have to agree with your
>>assessment.  There was clear digital clipping with source-direct off when
>>the input level was at maximum, but no digital clipping when source-direct
>>was turned on -- which proves almost conclusively that analog sources do
>>*not* go through A/D and D/A conversion with source-direct on. 

>>Steve Kanefsky

Please be aware in the demonstration that I described above and that Steve
has verified that turning the input level control all the way to MAX is really
not the best way to do it.  I was trying to shorten the description a little 
bit by saying turn it to MAX.  Actually the best process is to use a steady 
tone from a CD test record for instance, (or a sinewave generator if you know 
what you are doing), and then advance the input level until the OVERload 
indicator lights.  Then advance the input level JUST a LITTLE bit more until
you hear the distortion. The OVER indicator lights about 1 dB prior to the 
actual A/D overload point.  You must be in the source direct OFF mode.
You can do the test without a steady tone if the music level is fairly 
constant it just requires a little more care in adjusting the levels.
Now if you switch to the direct source ON mode, (this path bypasses the 
A/D -> D/A) then the overload distortion will cease. You can advance the input 
level control even farther without distortion even though the A/D OVER light
continues to show that the A/D is indeed overloaded. This proves that the 
direct source ON path does not use the A/D.

However, it is possible to advance the input level control so far that if
a large enough signal is present at the input, distortion will once again occur.
This is because it is possible to overload the ANALOG buffer amplifier of the 
TA2000 if enough signal is present at the input and the input level control 
is up high enough.  Since it takes a lot more signal to overload this path 
then the A/D->D/A path you probably will be able to turn the input level to 
MAX and never experience this ANALOG overload. Obviously this was the case 
when Steve repeated my experiment. Also note that this analog overload 
condition will never occur in actual use because the input level control must 
be adjusted to prevent the A/D from overloading from your highest amplitude 
source.  Since the overload point of the A/D is far below this direct source 
ON ANALOG overload point this condition can never occur in use.

I hope this helps someone from falling into a possible measurement trap.  Also
be aware that if a signal generator is fed directly to the input of the 
preamp, it is possible to overload the input buffer amplifier BEFORE the
input level control. In this case distortion will occur in either mode of
source direct and the OVER light need not be on at all. This should really be
obvious to anyone skilled in measuring electronic instrument parameters,
(suppress the inclination to comment on ALL magazine reviewers), but I thought
I'd better say it anyway.

Greg Rogers




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