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Stereo DTT into mono TV



 
 
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  #21  
Old December 29th 06, 11:34 PM posted to uk.tech.digital-tv
Dave Plowman (News)
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Posts: 5,342
Default Stereo DTT into mono TV

In article ,
David Taylor wrote:
Place an infinite impedence between each output and the input.


This will provide infinitely more interesting audio from the TV.


Trying to take the bread from my mouth?

But you get radio from a FreeView box too, and there's still some decent
stuff on that.

--
*Vegetarians taste great*

Dave Plowman London SW
To e-mail, change noise into sound.
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  #22  
Old December 30th 06, 01:50 AM posted to uk.tech.digital-tv
Bill Wright
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Posts: 8,408
Default Stereo DTT into mono TV


"Dave Plowman (News)" wrote in message
...
In article ,
Bill Wright wrote:
I very often have to feed the stereo outputs of tuners, etc, into the
mono input of a modulator. I did this by commoning the left and right
channels. I have never had the slightest problem. I don't believe there
is any real need to trouble yourself with resistors, etc. I have to
confess that I am more of a pragmatist than a purist.


You're probably right. Most devices will have an op-amp with something
like 100R and 100F in series with the output to make it reasonably bomb
proof.


Well, I was a bit wary of doing this at first so I checked carefully for
distortion, etc, but I never noticed any. It could be that if I'd hed a test
tone on one channel and nothing on the other it would have been different.
Where the tuner, etc, has a mono setting I use that, but also common the
outputs just in case someone tampers.

Bill


  #23  
Old December 30th 06, 06:48 AM posted to uk.tech.digital-tv
Johnny B Good
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Posts: 858
Default Stereo DTT into mono TV

The message
from "Brian Gregory [UK]" contains these words:

"Bill Wright" wrote in message
...
I very often have to feed the stereo outputs of tuners, etc, into
the mono
input of a modulator. I did this by commoning the left and right
channels.
I have never had the slightest problem. I don't believe there is any real
need to trouble yourself with resistors, etc. I have to confess that I am
more of a pragmatist than a purist.


Yes this will often work but I've known cases where is causes distortion.


One particular case is where the outputs are fed directly via coupling
capacitors from emitter follower buffers.


The outputs are very low impedance when driving the normal high impedance
but when they are called upon to drive into the very low impedance of the
other output they do not have the same drive capability for positive half
cycles as they do for negative half cycles so you get distortion.


This is very true. An emitter follower using a simple resistor
'pull-down' will show signs of reduced negative voltage 'headroom' with
even quite modest loadings of 10k ohm. However, the clipping assymetry
at this sort of loading is just a few percent and generally acceptable
on short runs of 'transmission line' with low capacitance loading.

A much better way to create a class A emitter follower output stage is
to replace the pull-down resistor with a constant current sink. Assuming
the emitter follower stage has been biased to remove any no-load
clipping assymetry, you can guarantee clipping symetry down to a well
defined minimum (resistive) load (typically 2 or 3 K ohms on something
intended to drive 10K 'standard' inputs).

I did just such a modification on both the replay and record dolby
boards in an Akai GX630DB tape deck in order to gain another 2 or 3 db
headroom on the 6 or 7 db increase I'd gained by adding the extra 2
resistors and capacitor that Akai had deemed to be 'superfluous' in the
origional dolby design.

This defficiency of output headroom (a clipping level of a mere +8db in
the replay circuitry) _had_ been commented upon by the reviewer in a
magazine[1] when playing pre-recorded tapes but the defect such penny
pinching had introduced into the recording circuitry of reduced low
frequency clipping level had been missed (the reason for the clipping
level only effecting the bass frequencies in the recording chain was due
to the recording bass lift equalisation being applied _before_ the dolby
encoder board).

This penny pinching was the only serious defect in an otherwise
excellent piece of kit. It was only later, when I clapped eyes on the
classic dolby board circuit in an issue of 'Wireless World', that I
realised that what I had calculated to be the extra components required
to remove the clipping assymetry where in fact meant to be there in the
original! Talk about a confidence boost! :-)

[1] Unfortunately, I can't recall the name of the 'Hi-Fi' magazine of
the top of my head right now, but I'm sure I've still got that review
stashed away somewhere. If anyone is interested, I can probably dig it
out and name the magazine in question.

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

  #24  
Old December 30th 06, 07:01 AM posted to uk.tech.digital-tv
Johnny B Good
external usenet poster
 
Posts: 858
Default Stereo DTT into mono TV

The message
from "Bill Wright" contains these words:


"Dave Plowman (News)" wrote in message
...
In article ,
Bill Wright wrote:
I very often have to feed the stereo outputs of tuners, etc, into the
mono input of a modulator. I did this by commoning the left and right
channels. I have never had the slightest problem. I don't believe there
is any real need to trouble yourself with resistors, etc. I have to
confess that I am more of a pragmatist than a purist.


You're probably right. Most devices will have an op-amp with something
like 100R and 100F in series with the output to make it reasonably bomb
proof.


Well, I was a bit wary of doing this at first so I checked carefully for
distortion, etc, but I never noticed any. It could be that if I'd hed
a test
tone on one channel and nothing on the other it would have been different.
Where the tuner, etc, has a mono setting I use that, but also common the
outputs just in case someone tampers.


Bill


Since, in principle, outputs tend to be low Z and inputs High Z, you
want to avoid having two outputs 'fighting each other' and quite
probably creating 'overload distortion'. A safe way of mixing down to
mono is to use a resistor of minimum loading value in series with each
output before the 'commoning point'.

About the only way you can guarantee acceptable results by commoning
outputs directly into a single input is on older systems based on the
(long since deprecated) DIN standard of constant current sources feeding
into low Z inputs.

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

 




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