Originally Posted by GregorioM
Paul Frindle and Paradox Uncr8ted:
In TV broadcast we've never been too bothered about inter-sample peaks because the specs we worked to usually resulted in peaks no higher than -10dBFS. The new North American (ATSC A/85) and European (EBU-R128) specifications both specify new peak levels, expressed in true peak, as calculated by compliant meters.
Although of passing interest, we're not really bothered by the fine detail of what happens in the SRC or the reconstruction filters in DACs but our reputations and jobs depend on us being able to measure and confine our mixes to the loudness and true peak levels as measured by meters compliant with the appropriate specification. It maybe, after going into the fine programming detail of these meters that some bright spark notices they don't actually calculate true peak precisely. But to us, that's largely irrelevant, our job is to get through quality control, as determined by some intern running software with the same metering algorithms as we're using. Hence this thread, to discuss limiters which will brickwall limit to the true peak specifications of our meters.
BTW, if you're interested, the tech docs can be found here: EBU-R128 (website)
. ATSC A/85 (pdf)
Both of these are based on the ITU-R-BS1770 specification.
Yes - I'm aware of this stuff - and it sure is needed because currently the user experience of sound conformity in broadcast is all over the place - even on the BBC! Not only do we have wildly varying levels between studio and OB reports and programme to programme inequality, we suffer over-loud music interventions in documentaries and drama. Even in the BBC 10 o'clock news, the level is around 2-3dB higher when it switches over to local news half way through!! These days I spend most of the TV watching time with the remote in one hand varying the volume.
This is what the ITU stuff is designed to ameliorate. The 'loudness' is computed by taking the RMS of a pre-filtered (LF roll-off) version of the incoming signal - and this conforms quite well to what we experience. It's little less good at music programme that may have undergone heaven knows what stuff to make it louder in the loudness wars, but it's still much much better than peak!! :-)
The meter itself gets this and shows the running peak 'loudness' levels over 3 timescales, using a gate to determine intentional silence, so that it can automatically discard an incomplete time block that contains silence and would therefore read low..
This model will not show you intersample peaks as such, but as you say, if programme remains at sample value peaks less than -6dBFS intersampling peaks are highly unlikely to occur - and maybe irrelevant to the intention of the ITU specs.
However we must still watch it? The reason we made our intersample peak meter/corrector is that my previous design in the Ox Limiter was not good enough to satisfy acceptance for some people producing broadcast material - on the very basis of intersample peaks themselves. The reason for this is that the OX limiter compromises slightly on freq content handling, reducing very slightly at 20KHz. This was OK 5 years ago, but current musical material is now so limited, squashed and maximised that it can breach the original design by a fraction of a dB.. So ths mattered to some people involved in broadcast at least? Remember, it is still possible to produce peaks above 0dB with programme at -10dB RMS - and even if peak limited to -1dBFS intersample errors can occur which are illegal samples that may not be decoded correctly by DACs, SRCs, data reduction coders and so on! The ITU meter will not
show you this stuff :-( We cannot ignore this stuff and simply blame DACs, in any case they are only a fraction of the story in today's signal chains - as you will no doubt realise.
Making a gain limiter (not a clipper) that conforms exactly to the ITU recommendations and controls loudness within specs is definitely doable, it is by no means a complex process - and adding the intersample stuff is easy to do. However, it would require a large degree of look-ahead latency (at least 400mS due to the max measurement block time in the specs) which would put the dialogue out of sync by nearly 0.5 seconds in real time..
Another thing worth considering is that the ITU refers to metering indication, not the programme itself! So if you applied the same constraints on the audio as specified in the visual meter read out, it would likely (definitely) be completely unacceptable in reality..!
So for anyone designing the compressor you are suggesting this is a very large hurdle to cross and will need to somehow work out the most robust design to keep it significantly below
the meter specs (so you never get an over), whilst maintaining an acceptable programme quality. Not easy IMVHO :-( In short it will be a large compromise and each manu will likely have a different set of compromises.
BTW - I should add that we can make you one, if it's a financially viable plug-in product..