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In contrast to production techniques applied to popular music, most published recordings of classical music can have a very wide dynamic range. This makes then less suited for listening in a car, on phone line, or other contexts where there is a lot of noise: if you were to boost the volume of your amplifier so that you could hear the quiet parts, you would get blasted when the dynamics change in the music.

When listening to my local classical radio station, I believe they are effecting the source recording to make it more appropriate for radio and listening in a car. I can recognize a number of things by listening to the noise floor of the track; namely, that when the music is very quiet (like a solo instrument in an orchestral piece), the noise floor is louder, indicating that the track has been boosted. But also, that when the full orchestra is about to loudly come in, the noise floor drops out just before the increase in dynamics actually hits, indicating that the boost has been removed.

How is this being accomplished? Is this some kind of compressor, or a predictive volume normalization algorithm of some sort, or is there just some engineer with their finger on the fader who knows the piece well enough to adjust the volume manually?

I would like to know more about this so that I can replicate the same technique on a collection of archival recordings of classical music for use as hold music for a phone system.

NReilingh
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Yes, it is simply a compressor - typically you'd use one with a relatively low threshold and ratio.

And some may store metadata for the whole track once it has been compressed/normalised once. I know my car stereo could do that - would zip through new tracks to identify peaks and normalise against them.

You don't need predictive normalisation though - remember you typically transmit with a 2 to 5 second delay, which gives plenty of time to use compression or indeed sidechain ducking for voice overs.

Rory Alsop
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It's 'simply a compressor'... however it's a very specialised type of compressor.

There are probably others in this field, but the go-to name for radio compression is Optimod by Orban

Optimod is, to over-simplify, a multi-band compressor specifically made for radio transmission & includes specific timing & frequency compensation for the way radio broadcast works.

Tetsujin
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There is a standard that is used called LUFs.

There is more than mere compression involved.

LUFs ensures consistent loudness by considering peak and average energy and the amount of time they occur.

Google LUFs for more.

edwina oliver
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  • Googling LUF leads to a type of fertility syndrome. Care to define what an LUF is? – NReilingh Jan 26 '20 at 21:27
  • Goggle gave me this as the first item: LUFS is a term for Loudness Unit Full Scale, which allows the measurement of loudness of a piece of audio without a reference, whereas a decibel measurement requires the reference of standard air pressure, against which to measure the air pressure generated by the audio. you need the s on lufS. – edwina oliver Jan 26 '20 at 21:28
  • And yet another cowardly stalker downvotes an absolutely correct answer. All TV and radio stations are using LUFs or similar standards to avoid the loudness wars as well as commercials being too loud. NPR has a tutorial on it. – edwina oliver Jan 26 '20 at 21:30
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    And yet the answer is still wrong, doesn't address what the OP asked for and obviously comes from a place where the writer knows very little about the output chain and compression chains used in radio stations but has probably spent 10 minutes googling "LUFS" which means it's going to appear in every single answer. Just be aware any up- or down-votes you get are not from "cowardly stalkers" they are from community members that are voting on the usefulness and quality of your answers. Nothing more, nothing less. My recommendation is to stop taking it all so personally. – Mark Jan 27 '20 at 22:57
  • The answer is totally correct and adds other valuable info. personal attacks do not change my 50 years in audio nor my knowledge of how thing work. – edwina oliver Jan 27 '20 at 23:11
  • edwina - there have been no personal attacks. Upvotes and downvotes are up to individual members of the community, and I would advise you to stop being aggressive about them or to commenters who try to help. – Rory Alsop Jan 29 '20 at 20:48
  • downvoting correct answers feels very personal. i respond the way i am treated. i see little real help but a lot of bullies with inferiority complexes who enjoy doing what they do. – edwina oliver Jan 29 '20 at 20:51
  • Your styling of LUFs with the lower-case 's' was confusing here, which is why I interpreted it to be a pluralization and got to the wrong google result. And given what the real definition is, I still don't understand why it's lowercased in your answer. – NReilingh Feb 03 '20 at 00:15
  • Speaking as OP, I greatly appreciate your input since it provides new information that was not mentioned before. Speaking as a former mod, I will also tell you that on this and most other Stack sites, 50 years of experience alone does not get you upvotes and reputation — being skillful in composing well-written and thorough answers that share your expertise does. People downvoted you because they interpreted your answer as incomplete or not helpful.

    I would be very happy to see you engage with this community by revising your answer with an edit to make it more thorough. That's up to you!

     – NReilingh Feb 03 '20 at 00:28