when i hear a song i already know on the radio, they always sound very strange to me. i'm not talking about bad FM quality or something. i think that the radiostations themself do something to the signal before sending them. to me it sounds like a strong compressor and some sort of stereo image enhancer.

does anynody know something about that ?

Methinks you ARE talking about FM quality.

Yes, I think the radio stations do do further dynamic range reductions (as if it is possible with the majority of music played on radio's), but you are predominently asking "How come I can recognise my mate Dave? I'm not talking about the fact he is as bald as a coot, I think there's something different about him..."

Dynamic range compression (nearly always), tone adjustement (sometimes), speed increase (rare)

As for Stereo Image, it depends a lot on the actual transmitter/receiver hardware. Because of the way stereo FM works, a lot of factors can change the stereo seperation, including the way the reciever is designed. There is a 'correct' implementation, but most tuner manufacturers don't do it that way.

The best you can expect from FM stereo is "it's in stereo" and not a particular level of seperation.

Good FM stereo is exceptionally good. Compared to CD it has lower bandwidth and stereo separation, also higher noise and distortion - but it's still more than good enough for most music for most people. It's less damaging than 128kbps mp3 for all music with a dynamic range under 70dB.

Unfortunately someone invented the optimod processor. This, or something similar, is used to squash the output of most radio stations prior to broadcast, in order to make them as loud as possible, and to ensure a constant loudness level which is always audible above the car engine.

BBC Radio 3 in the UK is still sometimes broadcast without processing, and the UK BBC national FM network is designed to give 99% of the population excellent reception with a roof top aerial. Most people don't use a roof top aerial, but this 50 year old system still works very well. (unlike its digital replacement!).

There are some samples here:
http://www.digitalradiotech.co.uk/digital_radio_samples.htm

The right hand column includes 4 FM samples. These were made using a single element indoor FM aerial and a £90 tuner. The pop stations show the typical processing, but listen to Radio 3.

Cheers,
David.

Lev certainly has a way with words...

i always swore that some songs seemed faster when they were played on the radio, but my family just thought i was crazy.

I notice that on the TV sometimes. A music video is on, i play the same song from my PC, and it often loses sync after some 20, 30 seconds. Now, i don't know if this is the PAL speedup for some NTSC tapes they get or what else, but it's definitely there. The funny thing is that the pitch isn't altered, unlike with many movies i've seen, where the pitch of the PAL version on TV is about one halftone higher than in the original NTSC version. Even more annoying when you know that it would be very easy to preserve the original pitch during the NTSC -> PAL transfer and still speed it up to match PAL speed.

as for stereo FM:

the signal
M + 38kHz_sine * (S+offset) // (amplitude modulation by S of the 38kHz carrier)
is sent via FM as one channel.
where M=L+R and S=R-L (or something similar)

it's somewhat backward compatible to mono receivers.
stereo receivers detect the amplitude of the 38kHz carrier
and try to restore L and R.

bye,
Sebi

Easy? I thought this would require intensive processing (and possibly introduce distortions).

Then again, a simple speed-up isn't as trivial nowadays as it was with analogue sources.

That is what Joint Stereo does.

No it's not that hard. I know that several movies have been pitch-corrected for PAL (Chicago, LOTR, Star Wars...). It's just another case of "the customer never complained before, so no need to do it".

I am often very surprised just how good it sounds. But it depends a lot on the reception and the tuner. One old boom box I had emphasised the side channel so strongly that the stereo sounded very artificial (like the singer is torn in half, rather than in the middle). It also didn't have a notch filter on the 19kHz pilot tone, which tended to annoy my dog.

Some good reading material on FM stereo (for those interested):
http://www.smoke.com.au/~ic/mpx.html


Only FM stereo doesn't enjoy the number of naysayers that JS does.

I'd say in the US, equalization is almost ubiquitous. Nobody but nobody sends their dynamically compressed songs out through a flat EQ, they always boost the bass and often several parts of the treble. As I understand it this is in part to make up for possible deficiencies in the frequency response of stock car speakers.

Certain public radio stations are the only possible exceptions I can think of. KCSM might not be messing with the tone.

to add to what Audible! said.. certainly nearly every FM station in my area has a huge amount of dynamic compression and boosted bass..

When I listen to their big, fat, ruined sound quality, I find myself wishing the FCC would mandate a standard apparent loudness level for radio brodcasting. They already dictate what the peak signal strength a station puts out may be, why not also regulate the average signal strength?

Creating a maximum allowable average loudness level would basically mandate a certain amount of headroom. You simply put the max allowed peak 14 db above the max allowed average, and boom.. stations playing dynamic recordings sound better than stations playing compressed recordings.

This would also create a fair playing field for small public and college radio stations, which can't afford the exorbinant Orban sound processors that 2Bdecided linked to above.

Maybe it even leads to record companies and mastering engineers creating dynamic recordings because they'll sound better on the radio! Oh well.. I can dream...

With the DVD player software on my laptop you can just say what time you want the movie to end and it will automagically adjust the playback speed and within some surprisingly wide limits keep the sound at proper pitch.

A talk show I listen to occasionally seems to have a processor that recognizes word gaps and moves them closer or farther depending the actual amount of material so the show will be the right length, though I wouldn't put it past the commentator (Harry Schearer on "Le Show") to be doing the word gap management in his head.

Back to FM, a good tuner (fed good program material) can give performance that would be difficult to distinguish from CD in many listening environments. 80 dB vs ~100 S/N ratio, but a refridgerator in the next room will wipe out that difference.

I suggested almost exactly this during the creation of a new UK broadcast regulator...

http://www.david.robinson.org/commsbill/index.html#2

...of course it was ignored!

Would have been nice though.

Cheers,
David.