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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 27th Dec 2016, 12:06 pm   #1
toshiba tony
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Default FM query

Years ago I could grasp the concept of FM stereo but nowadays I struggle. Sum and difference and the 19khz circuitry to provide left and right channels but I am wanting to know what happens circuit wise when you select mono. I'm sure there is more takes place than merely bridging the outputs. The reason I am asking is I have found whether you select mono on the tuner or on the amp the hiss is eliminated. Many thanks.
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Old 27th Dec 2016, 12:24 pm   #2
GeoffK
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Default Re: FM query

The 19kHz pilot tone transmitted to receive stereo is doubled to 38kHz second harmonic in the receiver and then using sum and difference to produce the two channels of stereo information. The 19kHz pilot tone is ignored or muted when mono is selected, and as the mono bandwidth is below the 19kHz pilot tone there is less treble and less hiss.
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Old 27th Dec 2016, 1:02 pm   #3
G8HQP Dave
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Default Re: FM query

Circuit-wise there are various options for selecting mono, although all are likely to result in less audio noise.
1. combine the two stereo outputs - may be used on a progressive or frequency-dependent basis if mono is required because of low signal strength
2. bypass the stereo demodulator/switch so just treat the signal as mono.

In the second case you just have the mono noise up to around 15kHz.

In the first case what you are relying on is that the noise in the two stereo channels is, to a significant extent, antiphase because it comes from the L-R difference signal. Adding the two channels makes this noise cancel, and you are just left with the L+R mono noise.
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Old 27th Dec 2016, 1:22 pm   #4
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Default Re: FM query

The difference signal, which is carried by the 38kHz subcarrier, is much more prone to noise than the sum (mono) FM signal, so it's the source of most of the hiss.

Selecting 'mono' on either the tuner or the amplifier removes this difference signal by forcing both channels to be the same. Because the hiss is in the difference signal, it disappears too.

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Old 27th Dec 2016, 2:04 pm   #5
vidjoman
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Default Re: FM query

The hiss will most probably be due to lack of aerial signal strength. You need a much stronger signal and the hiss should disappear.
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Old 28th Dec 2016, 3:19 am   #6
toshiba tony
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Default Re: FM query

Thanks, each answer clarified it, back in the 70's I did C&G one through to four, was it 224, that long ago now. Things have gone a tad blurry.
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Old 28th Dec 2016, 5:47 am   #7
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Default Re: FM query

This thread might provide some additional background:

https://www.vintage-radio.net/forum/...d.php?t=112156

Cheers,
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