Information over FM vs other cable

[JHZR2]

I'm not talking broadcast FM, rather taking a lossless audio clip, and injecting it over the FM connection into a radio. Kind of RF modulation, but taking any broadcast signal to zero and using the antenna cable as input to the headunit.

I'm doing this, and have to say it sounds great to me.

91 BMW 318i, OE headunit, MB quart component 5 1/4 in front and 5 1/ 4 rears (2-way).

Tweeters upfront are in the sail covers behind the mirrors (OE location). All through a good LOC to an MB Quart RAB 450 amp.

Clear, controlled, precise and sound from mirror to mirror. Great soundstage, great clarity, even piano sounds really good.

Most surprising is the bass. Imagine a song like pusherman by Curtis mayfield, with the sparkling sounds in the highs going from side to side, and then a nice, muddy, musical baseline. Stuff like Frankie valli, which has a good musical baseline plus everything else sounds so good.

When you get a bit harder, like I can't Stand It by Clapton, too loud the bass distorts a bit on the hardest notes, but awesome all the same. You can feel it.

Big subwoofers are so imprecise, and add too much thump for the mid bass type sounds which are most important to me. Don't think I'd even consider a sub if 6.5 speakers can be used in any application properly.

I'm just super happy.

Question is, for lossless music, how much am I loosing in a wired fm signal versus something else? Technically I'm sure it is a good amount, but practically, I doubt it is a huge difference from listening to the original lossless signal through $300 headphones...

 

[dparm]

How much shielding do those cables have? From what I've seen, the antenna wiring is pretty thin and barely protected. I wonder if you could just replace it with an RG6 that is heavily shielded.

 

[JHZR2]

Does it matter?

My device, when powered on, takes the fm sound floor to zero, and the wire going into the headunit is extremely short.

I'd imagine it would ultimately make sense to do this. But I'm not broadcasting, just using the fm tuner.

I suppose the question is if the bandwidth of an fm tuner limits signal versus, say, a 1/8" headphone plug aux-in on other headunits.

Because if so, I can't say that I hear it...

 

[dparm]

For a short run I doubt it makes any difference. If you had 100' of wiring or had some ridiculously noisy stuff nearby, that's a different story...

 

[Win]

There will be limitations to both the bandwidth and the dynamic range, that may not matter in a practical sense.

There is a 19 KHz pilot signal that has to be guarded, so the top end limit is probably 17 or 17.5 Khz, depending on your receiver. I don't know what your bottom limit might be, 50-100Hz would be a WAG.

If you are getting bass distortion you may be overmodulating the device, try reducing the audio level to your modulator. That would probably increase your dynamic range, also.

 

[Garak]

Isn't something like the signal to noise ratio of the FM receiver more of an issue than the wiring itself?

 

[JHZR2]

Originally Posted By: Win
There will be limitations to both the bandwidth and the dynamic range, that may not matter in a practical sense.

There is a 19 KHz pilot signal that has to be guarded, so the top end limit is probably 17 or 17.5 Khz, depending on your receiver. I don't know what your bottom limit might be, 50-100Hz would be a WAG.

If you are getting bass distortion you may be overmodulating the device, try reducing the audio level to your modulator. That would probably increase your dynamic range, also.


Ive only gotten distortion (which may actually be a rattle, cant tell yet), on one song. Some older stuff that wasnt well recorded, and where I can hear abnormalities on any system, can be heard too... But that's not the system's fault...

 

[Stu_Rock]

Win's got it. I think 15 kHz is a safer guess as to the bandwidth. DC is the lower end in principle, but equipment probably filters it out somewhere in the 10-60 Hz range.

Overdriving the modulator input is always a concern. Clipped signals have "corners" in the time domain that put power in higher frequencies.

Good equipment on both modulation and demodulation should be very high-fidelity for sounds lower than 15 kHz.

 

[JHZR2]

Seems to be!

 

[cchase]

Depends on your age whether the loss above (I'm going off memory here) 16khz is audible or not. I think that's the biggest fidelity issue. Should be a pretty good s/n ratio otherwise.

 

[digitalSniperX1]

Typical S/N in fm receivers is about 50db whereas a CD has 90db or more. The lower end is 50 hz. Having said that the best FM receiver can't fix what the transmitter messes up. Knowing all that...I once ownEd a Concord HPA-110 am/fm cassette Car sound system. On a specific station, when sitting in the car and quiet outside, I could hear the ever so slight turntable rumble in the background. That was a great station and a great receiver...all analog. And numbers aside, it was a great sounding combination.

 

[JHZR2]

Originally Posted By: digitalSniperX1
Having said that the best FM receiver can't fix what the transmitter messes up.


Well lossless encoding of my sources should minimize that. So hopefully Im as close to theoretical as is practical...

 

[JHZR2]

Originally Posted By: cchase
Depends on your age whether the loss above (I'm going off memory here) 16khz is audible or not. I think that's the biggest fidelity issue. Should be a pretty good s/n ratio otherwise.


This is a car, so ear quality or not, Id imagine that a lot is lost due to road noise... This isnt critical listening, it is just trying to understand what I get and how much practically it makes a difference...

 

[MolaKule]

Quote:
If you are getting bass distortion you may be overmodulating the device, try reducing the audio level to your modulator. That would probably increase your dynamic range, also.


I assume by lossless you are speaking to modulating a FM transmitter device with a pure analog signal?

In FM, the amplitude of the modulating signal increases the bandwith.

Unless you pass the modulating signal through a limiter that limits all frequencies, you cold be overmoduating the carrier on the bass and exceeding the passband of the receiver.

If you are not using a compressor/limiter, the only alternative is to lower the amplitude of the modulating signal.

 

[JHZR2]

Originally Posted By: MolaKule
Quote:
If you are getting bass distortion you may be overmodulating the device, try reducing the audio level to your modulator. That would probably increase your dynamic range, also.


I assume by lossless you are speaking to modulating a FM transmitter device with a pure analog signal?

In FM, the amplitude of the modulating signal increases the bandwith.

Unless you pass the modulating signal through a limiter that limits all frequencies, you cold be overmoduating the carrier on the bass and exceeding the passband of the receiver.

If you are not using a compressor/limiter, the only alternative is to lower the amplitude of the modulating signal.



Lossless in that I am pulling music tracks from their original source (CD) in a lossless format, that keeps all the data. No compression and no cutting off of "inaudible" frequencies, like an MP3 would.

I then play that off of, say, an iphone, through a 1/8 headphone jack (first source of "data loss", if the bandwidth capability of a 1/8" headphone jack input is not the full musical information), through a wired RF modulator (second source of loss, but this wired type blocks the OTA FM signal, so it can put all the data that it is being fed, up to the limit of FM bandwidth or whatnot at 88.7 MHz) into the antenna plug on my radio. From there the tuner (Third source of loss, if the tuner limits bandwidth or frequency information from the music) spits it to the amplifier (which I assume is more than full range), which outputs it to the rest of the audio system.

It sounds great, question is if FM losses in this manner of input are substantial, at the various stages.