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Old 16th August 2011, 12:42 PM   #31
forr is offline forr  France
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Quote:
Originally Posted by DF96 View Post
Sample and hold stereo decoders can run into problems with the sinc frequency response, which can give an HF rolloff.
That was EC8010 thought, see posts #15 and #17 :

Hybrid FM multiplex decoder?
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Old 16th August 2011, 01:19 PM   #32
DF96 is offline DF96  England
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His original idea was a switching decoder, but then the conversation shifted to sample-and-hold. It is possible that some people thought they were the same thing - I'm not sure. Both decoders have problems, but different problems.

Sample-and-hold suffers from a sinc frequency response, which is phaseless. Unfortunately this means that any attempt to correct it will introduce phase errors. On balance, probably not too bad. I saw a circuit somewhere (probably Wireless World or HiFi News) which used this method without any correction. I suspect the author did not realise what was happening, and may not have realised that the 'smooth' sound he got was due to HF rolloff.

Switching decoders give incomplete separation, because a pi/4 creeps into the maths, so a bit of cross-mixing is needed to cancel out the error. Decoder chips will already include this, but some DIYers might not realise that it is needed. You get 19.4dB separation without the extra cross-mixing, which will sound reasonable but measure poorly.

Interestingly, you could make a half-and-half decoder which gates for 13.16us instead of 26.32us but still at 38kHz rate. This would give a little bit of both problems, but perhaps not enough of either to worry about?
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Old 3rd April 2013, 08:38 PM   #33
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If you want to get into the nitty-gritty of fixing tuners, I hear good things about ""The Theory Of Servicing AM, FM, And FM Stereo Receivers" which is discussed in the thread by the same name:

http://www.audiokarma.org/forums/sho...d.php?t=495126

You can DIY a very good chip-based FM decoder and use it with any number of tuners including tube units:

fm-mpx : FM Tuner MPX Kits

I have one and it works well and sounds good!
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