Hosts: Kirk Harnack, Tom Ray, and Chris Tobin
Guest Bill Putney tells us how he put together a new community FM station. From equipment selection to installation to testing.
Guest: Bill Putney
TWiRT is sponsored by Telos Systems.
TWiRT 86 - Bill Putney Builds a New FM Station
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Been thinking about your comment on the need (or otherwise) for some form of protection in front of an STL in a digital system.
Here's my AUS$0.02 worth...
If we assume that modern processors have a good enough front-end AGC to not need a pre-processor, and that the STL's bit-depth (dymanic range) is at least equal to the rest of the system, then I don't think a compressor/protection limiter at the STL input serves any purpose.Where protection is needed is at the point where manual gain changes can occur, such that the "signal" can try to go past 0dBFS and hit the clip point. That'd typically be the mixer engine/surface.
Given the wide range of level "standards" adopted even pure digital sources (AES or IP) are probably not all going to hit the mix engine at the same "level" . Operator adjustments to compensate for that can result in inadvertent digital "overs" - but a post-mix engine protection compressor/limiter cannot fix this. It's already clipped. If it's "over" it's over!
Maybe we need dynamics control (limiting) on the mix bus of digital mixers to ensure they never generate digital "overs".
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Re: digital clipping:
I think you have to differentiate between a compressed (bit reduction) STL vs. an uncompressed STL.
If you're running an uncompressed STL, one that outputs bit-for-bit the data you place on it's input, there's really no need for input protection. Throw a fistfull of ones followed by a fistfull of zeros at it's input and that's exactly what you'll get at the output. The device following the STL sees the same waveform as what went into the link, nothing more or less.
But all bets go out the window if you're running a compressed STL. In this case, the compression algorithm is evaluating the incoming bits as audio and throwing out lesser sounds that fall underneath a mask surrounding a larger sound. The simplified audio is then passed to the other end, making the audio that comes out only an approximation of what went in.
Throw a fistfull of ones followed by a fistfull of zeros at it and the compression algorithm goes out the window with widely varying results. The algorithm thinks it's dealing with a large square wave and sets the surrounding masks accordingly, both in the frequency and time domains. You'll wind up throwing out the baby with the bathwater, discarding not only events occuring at the same time as the overload, but additional information occuring before and after the event.
In this case, you're much better off handling an overload in an upstream processor that's designed with these waveforms in mind. It can transform that overload into something more acoustically pleasing, at worst case limiting the damage to the duration of the overload.
In short, there's no real need to protect the front end of a bit-for-bit STL. You'll get out what you put into it. But bit reduction links should have overload protection in front of them.
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