Stereo Tool

Hi All,

We have a high powered FM transmit site which is elevated and can be received almost 100klms from the main transmit location.
We'd like to place repeaters / translators at different locations, on different frequencies, to rebroadcast the same program via off-air feed.

This is to address towns which are behind mountain ranges where we can't reliably get a signal into about 5 locations.

Physically I know it's possible to run a software (or hardware) based FM tuner that feeds into StereoTool, which can then be tidied up and RDS added before we rebroadcast via MPX. I have played a little with that combination.

My question is around sound quality - because we are going to rebroadcast an already broadcasted signal, is there anything special we need to do in order to produce a clean sound ?

My thoughts are to run as little processing as possible (the primary signal is already processed) but should I use the new (ignore high frequencies) functionality to exclude processing signals above 16KHz and Remove FM Hiss ??

Happy to hear thoughts from the group.

Many thanks


BeDazzler.

Hi BeDazzler,

Ignore high frequencies is only there to reduce CPU load; you can enable it if you like but it won't affect the result.

Is the main signal processed with Stereo Tool? And are you using the Composite Clipper there?

I'm asking because if yes, clipping again with a new tidied-up RDS signal will basically kill all the spikes above 100% (in fact, potentially already above 86%) that the composite clipper has let through. Which would be bad, obviously.

If possible I would just capture the MPX signal and rebroadcast that. But you need good reception and a good (flat from 0 Hz upto 60 kHz) receiver for that.

I would make nice WiFi network between sites and use some IP codecs to send unprocessed audio from studio.
Ofc on every site is box with ST.

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Note: If there are many people who want to rebroadcast FM signals and clean them up I can take a look at it.. Ideally, it would do this:
1. Split audio (left/right), pilot and RDS
2. Clean up the 3 signals. For example, for the L/R audio I would suggest using 'FM stereo hiss removal'.
3. Put things back together again, with proper pilot/RDS alignment.
4. Clip the result (should have nearly no effect, but some clipping is needed to get rid of spikes caused by reception problems)

In case of Stereo Tool composite clipping at the main transmitter, this would keep all the >100% modulated peaks, and give you a very clean signal which helps to improve reception when the rebroadcasted signal gets weaker.

I would prefer MPX tuner, anything else in the chain would probably drastically change audio quality.
Possible solution is to go with wifi links and exstreamers on the end (and then use ST), but that will cost some money and delay as well.

This what our chain looks like (complicated terrain - hills etc.):
STL (1.5GHz) -> nearby FM transmitter -> MPX tuner on higher location + 2nd link (1.5 GHz) -> another FM transmitter on different location (50 km away)

We get overshoots but not so much that we should consider ST on transmitter sites.

Regards.

Thanks for the replies and info.

Sorry for the delay, I have been away putting up wireless links.

The original FM signal output is processed by an Orban... it's not harsh and sounds clean on-air.

I have captured the FM feed via PC Tuner card and streamed into an AAC link to another location which is the translator / repeater site.

At the repeater site we run StereoTool with most options turned off, except...

- FM Hiss
- Stereo - Azimuth Correction: Enabled
- Limiting and Clipping - Hard Limit Output: Enabled
- Limiting and Clipping - Advanced Clipper: Enabled, +5.11dB (firm, no distortion)
- Limiting and Clipping - Highs and De-Esser: +3.56dB, 25% Slope
- Limiting and Clipping - CPU - Strictness: 20
- Limiting and Clipping - Sound - Sparkling Highs: 100% Strength, 50.0 Brilliance

The output of StereoTool is then fed via MPX @ 192KHz into the FM translator transmitter unit. The quality is surprisingly good.
No overshoots.

The only thing I notice is the high frequencies are not as clear as the direct FM off-air feed... happy to hear suggestions to brighten them up a little. I have tried not to process or touch the highs to keep it clean.

BeDazzler.

That could very well be caused by the receiver that you're using - I've noticed that many don't have a 'perfect' de-emphasis curve. By the way, you do have pre-emphasis enabled??

Azimuth correction in combination with FM hiss might not work as good as it could - it will first correct AZIMUTH, then fix the FM stereo hiss, but the hiss might not be in 180 degrees phase anymore. So I would turn AZIMUTH off.

Hi Hvz,

Thanks for that.

Yes, I am running FM output with pre-emphasis, RDS, etc enabled. which then feeds MPX.

I turned off Azimuth correction which does improve clarity.

I enabled Advanced Clipper -> Simple Clipper to tidy up the final output.

Quality is reasonably good, but I will search for a tuner with MPX output.... so if anyone knows of good ones please call out - preferably PC based would be good.

Cheers

BeDazzler.