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Old 30th August 2010, 06:38 AM   #5901
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Thanks for the input, hhoyt. I would not know the specifics, except for what my ears tell me. It IS amazing how this little tuner can 'lock in' to an FM station and stay there, without any obvious change from multipath, etc.
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Old 30th August 2010, 06:54 AM   #5902
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Quote:
Originally Posted by hhoyt View Post
Hi Pavel!
Don't let me put words into your mouth, but I suspect you were making an overall generalization, and one which would not be inaccurate. In my experience, here in the US 95% of all FM broadcasting (virtually all commercial stations) are processed to achieve maximum loudness by multiband compressors and other 'processors' which massively distort and spectrally skew the sound into an unrecognizable mess . To be the loudest on the radio dial is to attract the most advertising revenue, or so the theory goes.

HOWEVER...

The best non-commercial stations have high-end audio signal chains which can acheive over 100dB s/n at the input to the stereo generator or STL. These stations employ minimal if any compression, and use only an overall long-term time constant AGC to control deviation. With the best tuners this approach can result in an overall s/n of better than 85dB at the receive end. This can be subjectively 6-10dB quieter than a signal with the same single figure s/n rating, but with a flat noise shape, due to the HF de-emphasis in the receiver.
Hi Howard,

thank you for the explanation. This is a very good news for me, as I have always experienced here some kind of dynamic compression and short-term AGC. For this reason, I have never been satisfied with listening of classical music concerts through FM. In case that you have 5% of FM broadcasting stations who do not use any or use only minimal dynamic compression, and add only long-term AGC, lucky are the sophisticated listeners who can make a choice of such broadcasting stations.

Best regards,
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Old 31st August 2010, 02:53 PM   #5903
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John,

Maybe it is just you are listening with the wrong speakers. These are what I am using.

ES
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Old 31st August 2010, 11:40 PM   #5904
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Default Best filtering of FM chip output

I have the original RadioXTuners modifications to Sony XDR-F1HD. Stock rolled-off sound was not acceptible, nor was transition to inferior HD compressed subcarrier. Now I have solved those problems and others, this is definitely in the top rank of tuners, sonics included. Now I'm thinking of even better solutions.

Sony has a two capacitor passive filter between FM chip output and two transistor buffer. These capacitors have time constants equivalent to cutoff points at 150Khz and 75Khz. It is my guess that Sony is executing a low quality (and low Q) passive 50Khz filter. In my modified version, the filter is executed with "a hiqh quality chip" and typical lowpass feedback network, the the built-in filters and buffers are bypassed.

But is a two pole 50Khz filter the best solution here? What is the chip actually putting out here? My guess is that it's obviously highly oversampled if not a sigma delta source. I figure the main reason why there is rolloff at the top of the passband in the stock circuit is that the Q of the cheap filter is so low, something like 0.3 (though I leave calculation to circuit modelers). Raising the Q to 0.5 by having two identical buffered filters might be just enough to keep the passband rolloff below 0.1dB. So that's the modification I'm thinking of now. I plan to run the FM chip output through a resistor/capacitor lowpass filter with 100Khz rolloff and no buffer (cable capacitance merely parallels small 0.03uF cap in filter), then implement second 100Khz rolloff in a following tube buffer stage.

Now in typical SACD filters, don't they use more than 2 poles for 50Khz lowpass? That's what I was trying to find when I found this thread.

But tapping directly into the digital would be even better in my system, which runs on DSP for crossover and EQ. Is there high rate PCM or is it some kind of delta sigma?
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Old 1st September 2010, 12:25 AM   #5905
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FM rarely has content above 15 KHz. Analog FM has a 19 KHz pilot tone at 10 % modulation or so. The 38 KHz subcarrier also limits HF extension in FM, too much HF into the 38KHz and it folds down below 19 KHz and messes up the stereo decoder. A well designed low pass filter at 20 KHz would be more than enough.

The HD radio runs at 44.1 KHz sample rate (so at least most CD's won't suffer the abuse of bad sample rate conversion) but that again sets a max audio frequency of 20 KHz. HD radio is represented as essentially CD audio quality making still more reason to look at adding an SPDIF output.
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Old 1st September 2010, 12:50 AM   #5906
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Demian, please stop spreading tales about CD quality

from HD Radio - Wikipedia, the free encyclopedia
Quote:
Digital information is transmitted using COFDM with an audio compression algorithm called PAC Lucent's (Perceptual audio coder). However, audio compressed with this method led to complaints about poor sound quality, so in 2003, iBiquity combined it with an enhancer called SBR (spectral band replication) that improves audio quality at very low bit rates and branded the codec to HDC (High-Definition Coding). (HDC is a proprietary codec based upon but incompatible with the MPEG-4 standard HE-AAC).
usually we are listening 25 kbit/s MP3 like garbage HD1, HD2, HD3
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Old 1st September 2010, 02:00 AM   #5907
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Quote:
Originally Posted by 1audio View Post
FM rarely has content above 15 KHz. Analog FM has a 19 KHz pilot tone at 10 % modulation or so. The 38 KHz subcarrier also limits HF extension in FM, too much HF into the 38KHz and it folds down below 19 KHz and messes up the stereo decoder. A well designed low pass filter at 20 KHz would be more than enough.

The HD radio runs at 44.1 KHz sample rate (so at least most CD's won't suffer the abuse of bad sample rate conversion) but that again sets a max audio frequency of 20 KHz. HD radio is represented as essentially CD audio quality making still more reason to look at adding an SPDIF output.
In principle, there could be baseband mono audio in FM all the way to 22khz, which is the lower limit of the 38Khz L-R subcarrier. People often think that tuners which implement 19Khz cancellation sound better than tuners which implemented a 19Khz filter. Some tuners didn't even bother to include filtration of all the stuff above 22Khz.

Anyway, the Sony XDRF1HD tuner already filters out 19Khz pilot and processes 38Khz modulation. What comes from the chip output does not need a steep filter, which would have been required for either of those. All that is required is a gradual filter to cut the high frequency digital sampling noise, similar to what is done for CD where there is digital filtering or for DSD.

Just because the filter *can* be gradual, doesn't mean it needs to be. A steep filter at 15Khz would probably give the best technical specs, if not sound quality, because it would increase S/N and tuners are only spec'd to 15Khz anyway.

But since the tuner is already a bit challenged in the sense of high frequency openness, a steeper and lower frequency cutoff is not likely to improve sound quality. Instead, what is needed is a better gradual filter of some kind.
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Old 1st September 2010, 06:49 PM   #5908
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Wow, what a looong looong thread,

I still have a few pages of part 1 to read, but i thought I would share this patent with you:

US3054067

Just look at the filing date!!!
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Old 2nd September 2010, 03:43 PM   #5909
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Thanks for the input charlesp210. I remember the good old days of mono FM and I was even able to make quality 15 ips analog tapes from live events. Stereo was a bring-down, but MONO still could sound pretty good.
Stereo is a REAL compromise, and most of us listened in MONO, at least until recently. This little Sony is a REAL find, especially if we can FINISH it to hi fi standards. Apparently, serious FM manufacturers have tried to get the chips themselves, to no avail, from Sony.
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Old 3rd September 2010, 03:47 AM   #5910
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One of the biggest headaches in audio design today is getting the technology developed by the major manufacturers and being able to optimize it with better power supplies and output stages.
We can often buy 'one off' components from the major manufacturers, but then we have to bypass some of their 'lowest common denominator' circuitry, that is just added on to complete the unit, and here is where the audio quality suffers, if you believe in such a thing. When we audio manufacturers try to make a product, we are faced with BIG licensing fees from Dolby, Sony, etc. above and beyond just buying their technology that is relatively cheap to buy. It limits our ability to manufacture, and we are stuck with making amps, preamps, and loudspeakers. Sources such as CD, DVD, SACD, and even FM are limited to us.
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