Low(ish) audio & distortion remedy for TX

[MiXiN]

Alright necks,

I've got a few Transmitters here, and when I turn the modulation up to a level of reasonable intensity the audio goes distorted and really crap; I'm talking audio levels of only about 1/3rd the level of other FM stations - so it's not particularly loud.

I've fed audio directly to the Transmitter, fed it from a Stereo coder to MPX input, and the results are all similar.

The best way I can describe the result is that the audio sounds like the scrunching of crisp packets at certain peaks, and as aforementioned, the audio isn't overmodulating as it's only about 1/3rd the volume of all other stations on FM.

Does anyone know what the issue is, and a possible fix?

[shuffy]

Yes, you're overdeviating and the cure is to use a limiter! There are loads of subjective reasons why you might think you're not overdeviating because the rest of the audio is quiet, but you still are on the peaks. These range from your music source (particularly if you're using something like an ipod with a headphone amp on the output) to the content you're playing. Audio processing is important. Try compressing your audio file with something like soundforge and play it again using the same music source (and no "loudness" or other EQ settings turned on). You should be able to set the level for decent sounding modulation. If you can do that, then get a limiter. The Pira one is pretty good.

[pjeva]

Also it depends on the input of your transmitter, but most probably you're overmodating it as Shuffy said. Reason why you think you sonund lower than other stations is audio processing, and you must use some software or hardware compressor / limiter to overcome this. Try to find Breakaway Broadcast and use its internal stereo coder. With Plutonium or Amsterdam preset, you should sound almost loud as commercial stations.


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[shuffy]

Also it depends on the input of your transmitter

It could, but since Mixin said he's got a lot of transmitters I assumed he was getting the same thing with all of them, so I ruled it out.

[yellowbeard]

Try relaying a radio station with decent loud audio on it, they have already done the processing. If you can get levels approaching the original with no distortion then the problem is you need a compressor/limiter. If you can't get levels at or near the original station without distortion then the problem lies elsewhere and we will all be asking for more details.

[Albert H]

MiXiN: Audio processing for broadcast is one of the most misunderstood aspects of broadcasting. Over-modulation of an FM signal leads to interference to adjacent stations (a sure way to get an "unfriendly visit"), and renders your signal unlistenable. You've already experienced the gross, nasty distortion of an over-modulated FM signal.

Music is generally recorded to CD (or other media) with a dynamic range of 30 - 40dB. If you set the modulation depth on your transmitter for the peaks, the quieter bits will be down in the noise. This is why your signal sounds weedy compared to your neighbours.

The usual broadcast audio chain has a slow-acting AGC as the first processor. This raises the levels of very quiet stuff and is basically there to help even out the big range in the overall level of the source material. You then have to apply pre-emphasis - the treble boost that's a standard for all transmitters. All receivers are equipped with the corresponding treble cut circuit, and since most background noise is high frequency hiss, the hiss is significantly reduced.

The next stage will be a compressor - fast acting, to reduce the dynamic range from 40 dB to about 10 dB. It still has some amount of dynamic in the material (quieter bits are quieter than the loud bits), but the overall average has been increased. This will already make your signal sound much "louder".

The final stage is the limiter. This can be as crude as a clipper (Orban charge $1000s for glorified clippers) or as sophisticated as a delay line feed-forward limiter. Most stations go for cheap and simple, and as someone suggested earlier in the thread, the "Pira" one gets the job done. You can also buy a cheap Behringer "Composer Pro" limiter which is remarkably good for the price, which will get you most of the way there.

I always prefer analogue solutions to digital ones, but there are some excellent broadcast processors available in software. They're nowhere near as good as the Real Thing™ but they're a good approximation.

[MiXiN]

Thanks for the replies.

The audio source used was from the inbuilt music player of my Samsung Galaxy phone; on all other settings the music was just flat & hollow, and only when I managed to reproduce decent audio from the TX was it distorted.

Since posting this question, I did a bit of Googling - and as well as what has been posted here, a fair bit of stuff about "15Khz Brickwall filters" came up with various people suggesting that the high frequencies cause this "rustling paper/crisp packet" effect.

When I do eventually decide to set up and find a TX site, would one of these Brickwall filters & just a Pira limiter suffice? I want to keep things as minimalist as possible.

[pjeva]

Brickwall filter is low pass filter for audio which will prevent frequencies above 15kHz to pass into stereo coder and/or transmitter. It will eventualy reduce hiss and "high frequency roll-off" but probably would not solve your overmodulating issue. What are your plans for link between studio and tx site? If you are planning to send mpx signal, then it is very easy and cheap to process sound at studio side and deliver processed and compressed and limited audio to your tx. Also, you don't lose your audio processing equipment when your tx is taken off (by Ofcom or rig thieves)


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[NOYB]

....Plus the audio components in most phones are cheap and nasty, hence poor sound quality.

Get an old laptop, install the free version of Stereo Tool, select the "Unweil" preset, set the sound card sample rate to 192kHz and use it to process your audio, generate the stereo MPX and RDS signal. Then stick that in your TX (if it has an MPX input without 15kHz filters) and adjust deviation. If your sound card cannot to 192kHz, then you can "only" use Stereo tool to do the processing and will still need external stereo and RDS encoder. When set-up correctly you can sound as good as or better than most pro stations, and maybe even "Louder" if that is your goal.

[shuffy]

As a last resort, and only for testing, I've used phones as an audio source. The results are invariably abysmal. This includes the 2 Samsung phones I've had recently. I don't recommend using anything with a volume control on it to drive a transmitter. Invariably they are intended to drive a low impedance load, which you don't have, and increasingly with more "modern" devices there is processing to compensate for artefacts of headphones even when the response is set to "flat".

This is a very subjective area and because of that, discussions like this can end up getting quite complicated. To be as general as possible:

I think your crisp packet effect is overdeviation - your modulating signal is causing the carrier to deviate outside the bounds of a filter in the revceiver. Are you getting this on bass beats or sibilants (e.g. cymbals, hi-hat etc). Only you know the answer as you're familiar with your source material! Most commonly this happens on the kick drum.

Regardless of the above, if you don't have any audio processing, you need it.

As Pjeva said, the brickwall filter is generally to keep things within spec and prevent high frequencies aliasing and messing up the spectrum of your stereo multiplex, taking out the pilot tone, and other stuff you don't want. It's not necessarily related to your overdeviation problem but yes, ideally you need a low pass filter especially if you're in stereo.

[MiXiN]

Thanks for all the good input necks.

When I get paid in a few days Im going to heed the advice and first invest in one of those Pira CZ Limiters and use a different audio source.

I'll trial various things as I still don't have a TX site so have a fair bit of time on my hands.

Shuffy - forgot to mention - the distorted crisp packet scrunch effect tends to happen mainly on cymbals and other high frequencies.

[shuffy]

Shuffy - forgot to mention - the distorted crisp packet scrunch effect tends to happen mainly on cymbals and other high frequencies.

Sorry Mixin I didn't notice that last comment. Usually it's the bass, but it could be that you're using a passive pre-emphasis network and overdeviating on the high frequencies because you've wound the level up to sound louder on the lower frequencies. Proper compressing/limiting of the whole signal should sort it, and a LPF is a good idea especially if you're in stereo (if your stereo coder doesn't have it built in, which most do).

[MiXiN]

Thanks Shuffy.

I've not bothered with that Turkish Pira limiter since hearing bad reviews on here - so have ordered something better. I will post my results when it's arrived and connected.

My Stereo coder is just an NRG pro III, and I'm not sure on its specification as I've lost the paperwork it was supplied with.

[Albert H]

The Pro III coder doesn't have lowpass filters in the audio path. It was designed as a "cheap as possible" basic coder.

Stephen and I came up with a fairly simple lowpass filter unit that fitted on a board the same size as the coder. It was designed to fit above the coder, and would have links vertically between the two units. It's really simple, using four quad op-amp chips (TL074) and a bunch of capacitors and resistors.

I'll see if I can find the drawings for the filter unit and if so, I'll put them up here for everyone to use.

[radionortheast]

simple fix would be run mono, no rds either, maybe have a simple 15khz fliter if the transmitter dosen’t have one, run processor software, (like stereo tool) throught a soundcard not headphones. (I had really messy treble using the headphones out before switching to a soundcard, even thought the output from laptops by the headphones has got better don't be fooled)

You could use one laptop as a processor, runing the stand alone version of stereo tool to get audio at the right level from external source like an mp3 player, or relaying music off of youtube or soundcloud/mixcloud or something, tis very good, turning up and down the input you can tell if its working or not. don’t know if any of this helps…….

[shuffy]

You can also buy a cheap Behringer "Composer Pro" limiter which is remarkably good for the price, which will get you most of the way there.

I'm not so sure about that for broadcast radio (not internet) since you can't put the pre-emphasis at the right point in the chain!

[Albert H]

You can put passive pre-emphasis before the processor - I know several stations doing this, and they sound pretty good.

[shuffy]

That's something I'd thought about, but you can't be sure until you try it. I like active pre-emphasis because there's less to compensate for downstream, but I thought that with passive pre-emphasis on the front of a processor like the Composer or the dbx, that the expander + compressor will compensate for the attenuated bottom end and the limiters will sort out any residual artefacts due to their combined effect on the top end. I like the spec of the Behringer 2 channel unit (and I know you use one) but I've had issues with the build quality of Behringer's kit in the past so I might try the dbx this time. It's time to ditch all this software.

[Albert H]

I've never really had any problems with Behringer gear. I wouldn't take it on an extended road tour, but bolted into my 19" rack in the studio, it does what it claims to do at a reasonable price. I even use one of their mixers - somewhat modified! Their DJ750 is hard to beat at the price.

[teckniqs]

All FM 96.9 (CR station) in Manchester use a Behringer thing for their audio.
Looks a lot like this...