Hiss on FM stereo

[paulsherwin]

Yes, this is different. It sounds like some sort of multipath problem, which is quite likely if there are lots of hills.

[Nuvistor]

This may or may not be related to your problem with modulated hiss but it is an interesting read and not one I had thought about.
http://www.itu.int/dms_pub/itu-r/opb...2011-PDF-E.pdf

Frank

[Synchrodyne]

Thanks for that Nuvistor. I have noticed modulated hiss here on Concert FM on some receivers. Just below Concert FM (which comes from the Te Aroha regional transmitter, 50 km or so distant) is a local FM station of the very highly compressed, “shouting at you” kind. So the latter is likely the culprit. (And that is another reason, apart from its inherent ugliness, why significant compression is “bad”.)

On the main theme here, I have attached some “pictures” that might illustrate the points already made by others.

The FM receiver curves are actually those for the Revox A76, which in retrospect may be seen as one of the early solid state “supertuners”. The 20 dB noise debit for stereo at lower signal strengths is clearly shown. Also, the curves show that the conventional wisdom, that around 1 mV of “clean” signal is needed for good stereo reception, is not far wrong. Blending of the L and R signals – or otherwise reducing the stereo separation – would result in intermediate curves.

Re the triangular noise curves, something to bear in mind is that whilst the audio baseband is wide FM, with a modulation index of 5, the subcarrier is veering towards being narrowband. At 38 kHz, the modulation index is just under 2, and at 53 kHz, the upper edge of the subcarrier band, it is just 1.4. (Actually these numbers should be adjusted downwards slightly to account for the fact that the maximum deviation allowed for the M and S signals is ±67.5 kHz, not ±75 kHz, to allow room for the pilot tone.)

With narrowband FM (NBFM) systems, it is not unusual for 6 dB/8ve pre-emphasis to be used across the whole audio band, thus allowing receiver de-emphasis to fully offset the inherent triangular noise curve. When the maritime VHF-FM parameters were first established (see Wireless World 1957 April) the deviation was ±15 kHz (since reduced to ±5 kHz, I think). Against the 3400 Hz upper limit of the voice band (300 to 3400 Hz), this represented a modulation index of 4.4. 6dB/8ve pre-emphasis was used across the band. At ±5 kHz deviation, the modulation index comes down to just under 1.5. So one may see that the FM stereo subcarrier is at a disadvantage as compared with customary NBFM practice in that it has no overall pre-emphasis, although of course the AF signal impressed upon the subcarrier is pre-emphasized.

One may speculate that had FM stereo been developed about a decade later than it was, some form of noise reduction might have been considered for the subcarrier channel. Post facto attempts to add noise reduction, such as Dolby B processing and the FMX system did not take. But DBX companding was used for both the stereo subcarrier and SAP channels in the American MTS TV sound system developed in the early 1980s. I recall that some time back, one of the Japanese makers (I think) offered an FM tuner in which stereo decoding used only the lower sideband of the subcarrier in an effort to obtain a lower stereo noise level. I imagine that IQØ techniques were used to access the lower sideband, possibly building on what was developed for FMX decoding.

Cheers,

[Radio Wrangler]

The wheels grind exceedingly slowly, but that ITU-R report is likely to result in an ultimatum to broadcasters "If you want to use so much audio compression, then you must either reduce carrier power by this much, or cut your peak deviation by this much"

Either way, their 'loudness' goes down and you can foretell how popular that will be. But enforcement of these things is complaint-driven. So get complaining!

David

[Hybrid tellies]

Originally Posted by Timvintage

I've always wondered why it is that when you listen to a radio station in FM stereo, there is always quite a lot of hiss? For me, this is too annoying to listen to and I have to switch to mono. I always think to myself "what's the point in having FM stereo at all when there's this much hiss!" What I'm thinking is, is there something I can do to get rid of the hiss, or is it just the way it is?

You are lucky you can switch to mono which is not a common feature on more modern FM portable radios. Whereabouts in Bristol are you?
Where I live (Longwell Green) reception of the national stations from Wenvoe and Bristol Illchester Cresent are weak and variable making stereo reception on portable radios almost impossible. For good stereo reception a large 4-6 element roof aerial is required.

[BGmidsUK]

Isn't this "modulated hiss" what they refer to as a "birdie"? This can apparently be caused by signals which are too strong for the receiver to cope with, though reading the posts above this is unlikely to be the cause.


Brian

[G8HQP Dave]

My old Nelson-Jones tuner used to have a burbling birdy on Radio 3. I assume it was 3rd order IM from R2 and R4, due to the 2.2MHz spacing either way.

With sufficiently high local noise you could get both audio noise and birdies.

[paulsherwin]

Given Pete's location in the Pyrenees I still think that a combination of multipath and co-channel interference is the most likely explanation.

[G6Tanuki]

Originally Posted by G8HQP Dave

My old Nelson-Jones tuner used to have a burbling birdy on Radio 3. I assume it was 3rd order IM from R2 and R4, due to the 2.2MHz spacing either way.

One of the benefits of a directional antenna is that you can play around optimising the signal to get the best balance between what-you-want and what-you-don't.
Alas this technique doesn't work so well when both the wanted and unwanted signals are radiated from the same transmitter. I've sometimes found it better to aim the beam sort-of at a semi-local transmitter carrying the program I want if in doing so I can put the local source-of-burblies-and-the-program-I-want into a null in the antenna's polar diagram.
"Strongest Signal" is not always the cleanest signal.

[emeritus]

In the 1970's i knocked up a simple dipole for FM, cut to a nominal 100MHz using data from a book that I then had, from some 15mm copper pipe, forced into a short length of plastic conduit, and I am still using it. It has a sharp null that I could use to null out local transmitters when listening to French FM stations, possible back then before the spectrum filled up. The French stations usually came in loud and clear in noise-free stereo. I was then living in Chadwell Heath, on the eastern side of London

I was using a Alba UA700/UA800 setup, but the tuner couldn't cope with the BBC's RDS system, producing various noises as they tried different schemes, sometimes being similar to a diesel engine ticking over. The BBC couldn't come up with a fix for my receiver, so I then had to listen to the BBC in mono. My RDS interference doesn't sound like the problem at issue, as it was more of a throbbing noise in the background than a hiss. I just posted to confirm that the directional properties of the deep null of a simple horizontal dipole can be useful.

[Synchrodyne]

On modulated hiss and birdies, the case I previously quoted had both. The birdies are noticeable during the brief periods of no modulation. Either they are masked or drop away during modulation, at which time the continually varying hiss becomes noticeable. I suspect that both are caused, in part at least, by the nearby heavily compressed station.

Cheers,