Trade Chat Peak Program Meters

@hurty In my little bit of the BBC it's now RTW displays. Recently I rescued a meter block with real dual Sifam PPMs in it from a scrapped Studer Vista sound desk but regrettably, they're AES inputs.

When I was a teenager doing recordings on my Panasonic stereo cassette unit it had the VU meters. It was difficult to get information on how to correctly use VU meters or what they were really telling you, no internet then, and what a pain they were. Do you aim for a +3 VU peak or a 0 VU peak? Never knew what was correct.

I remember whilst sat at my bench one day in 1978 listening to the radio, would have been Radio 1 then, I was thinking I wonder how they connect the record deck to the mixer? would it be using phono's? and what level would they use? I had no idea but by January 1981 I had all the answers as I had joined the BBC and was on my 12 week technicians "A" course. One of the first lectures/demonstrations was an introduction to audio levels. This is were the Peak Program Meter (PPM) was introduced to us. There was tone fed to a VU and PPM meter together. The VU meter was set to 0VU for 0 dBu which is PPM 4. A VU meter is normally set for 0dBu = -4VU but there are two standards used! With steady tone the VU meter read 0 and the PPM read 4. The tone was then switch on and off at a slow rate. This rate of on/off being increased. What then happens is the PPM always reads 4 but the VU drops off in level such that it will read the tone level as half or lower. The tone level was always 0dBu. The VU meter was too slow in response time to follow the level of the pulsed tone. The PPM meter has a very fast rise time and a slow decay time hence it can follow the true peaks in audio. As a result I can look at a PPM meter and tell you how loud that is going to sound and if it is correct. There is an active circuit within the PPM giving it the required response and decay times

PPM meters are used across all British Broadcast stations as the standard. For the more technical amongst us, I can hear the shots being fired at me now, you have to go back into history a bit as the Post Office equipment for audio was always 600ohm input impedance to be matched by a sending impedance also of 600ohm. This was somewhat a pain as you could not bridge to two destinations. Over time in the 80,s this was dropped in favour low output impedance 10ohms or less into input impedances of 10kohm or greater. This made life much easier and and reduced the number of audio DA's required. Some DA's had low output impedances but one 600ohm output for feeding BT equipment.

Another difference of broadcast audio compared to domestic audio is domestic being unbalanced audio, screen and signal, the broadcast audio being balanced audio. This balanced audio is differential and requires 3 connections signal +ve, signal -ve and screen. Just point out that this works o.k. without the screen (ground). The test tone levels used in broadcast are 0dBu, PPM 4 which is 775mV RMS, 2.2 Volts peak to peak. To see this level you will need a scope with differential input or use two channels, inverting one then adding together. The reason for using balance audio is it is very noise immune and ground loops can be easily dealt with if needed. Domestic audio used on phono's always used to be -10dBu so a much lower level.

Coming through my broadcast audio experience when I started at the BBC and then Thames Television it was all mono audio. Next came Stereo Audio, this all being analogue, stereo with NICAM delivery. Next came digital AES3 audio. One balanced audio lead that carried one mono audio, in AES it carried a stereo pair at 48k sampling rate, made life a bit simpler. This AES audio could be embedded into Serial Digital Video, that is another subject. Since then along came Dolby E broadcast standard could carry 8 audios down one AES lead and with this Dolby D, six channel surround sound, moving on to Dolby Atmos which can be up to 16 channel audio direct to the customer.

Below I have a picture of the metering I use in the workshop. The top meter is a stereo pair PPM meters fed with unbalanced audio, broadcast standard.

The second unit which is a bit unusual is a PPM meter which uses unbalanced audio.

The lower meter being a VU stereo pair. This is calibrated at 0dBu reads 0VU

I have also put in audio metering scales that may help people understand the differences between different metering systems

If people want I can go into more detail like most of the divisions, not all, on the PPM are 4dB so as indicated on the top PPM pare they are showing +4dB = +4dBu = PPM6

Hope this provides some background as to how the audio around the station to you is monitored and delivered to you.

There is much more to this now as digital systems have a maximum signal by default, where all the audio sample bits are set to "1" being 16, 20 or 24 bit samples. The more samples the better audio resolution, no it doesn't get louder. In this country and Europe the maximum level 0dBFS= 0dB Full Scale. This is the EBU digital scale on the chart. We work to -18dBFS as our reference level so station tone will be fed around at -18dBFS which = PPM 4 = 775mV rms

Digital audio, reference level, is shown below on meters with a dBFS scale. Ignore the second stereo Dolby E pair which is full scale as the Dolby E shown here carries 8 compressed audios on a single AES pair and therefore is not representing audio level.

Hope this introduction helps with understanding audio. A complicated subject.

Any questions, do ask, I'll duck the bullets.

Posted by: @hurty

↑

There was tone fed to a VU and PPM meter together. The VU meter was set to 0VU for 0 dBu which is PPM 4. A VU meter is normally set for 0dBu = -4VU but there are two standards used! With steady tone the VU meter read 0 and the PPM read 4. The tone was then switch on and off at a slow rate. This rate of on/off being increased. What then happens is the PPM always reads 4 but the VU drops off in level such that it will read the tone level as half or lower. The tone level was always 0dBu. The VU meter was too slow in response time to follow the level of the pulsed tone. The PPM meter has a very fast rise time and a slow decay time hence it can follow the true peaks in audio. As a result I can look at a PPM meter and tell you how loud that is going to sound and if it is correct. There is an active circuit within the PPM giving it the required response and decay times

I remember that demonstration well and I'd also add that the meters themselves had specified ballistics. Yet VU meters persist everywhere.

Of course, this is the point where LUFS* enter the discussion...

*Loudness Units (relative to) Full Scale, which is an average measurement.

Hi Cathovisor, I was awaiting your response as I knew it would be coming. LUFS, whilst doing a demonstration to the head of engineering years ago of a PPM meter verses a VU meter, he is Australian and only used VU meters like the Americans, using the stepped tone source it became evident to me that a VU meter is in fact a very good LUFS meter. I did win my point that PPMs where superior. We have this new audio level metering in broadcast called "Loudness Units Full Scale" (LUFS). The broadcast level when you measure a source file be it a commercial or program or indeed a film is now specified in this country that it should be -23LUFS +/- 1. A LUFS meter ignores short duration loud noises eg like a gun shot and also ignores audio at low level. Again there are many different standards of the parameters and durations used! but this is similar to a VU meter as it cannot measure short duration "loudness". The main reason for the new LUFS measurement is it can be done in software looking at the audio tracks of content files, hence no people are required. In my experience it does not do a very good job. More of a problem in live sports programs as this is a standard that has to be met! so the sound mixer person has to keep an eye on yet another meter. As to why I am sceptical you only have to listen to a channel, channel 4 comes to mind, there are many others, where the adverts are considerably louder than the program content so you are always reaching for the remote volume control to turn the commercials down, yet these commercials pass the "Loudness" requirement as do the programs. Just a bit of background to commercials is they want to make an impact so make it loud, but you cannot have loud so they compress everything at 0dBu (they used to do it at almost +8dBu). Radio 1 works like this so there is no dynamic range in the audio!! You would see this on your PPM no problem, the meter never deviating from 5.5, just stuck there. Where I have worked we always put 6dB pads on the commercials and everything then came out correct, commercials matching program content. I could go on, and on. 

A LUFS meter is shown below monitoring channels 1&2. The LUFS reading should be -23 but is not as I'm using tone which is a test level signal at -18dBFS so does not meet the loudness measurement as it is not supposed to.

 Hope someone is finding this interesting?

@hurty I knew a sound supervisor (who shall remain nameless) who, when I asked him how he addressed the -23LUFS issue on his show said "I just set a limiter at PPM 5 and a quarter..."

I did think a VU meter would be better at showing an average, but that was never any good when unexpectedly loud peaks could do real harm to your transmitter's output stage in the past...

@hurty Interesting to read about sound levels from two who have broadcasting experience.

One of my biggest difficulties is actors who whisper and mumble so the volume is has to be turned up, then the adverts blast out and as you say quickly reaching for the mute control. I mute the adverts if they are intrusive so any message they have is lost.  Most of the adverts are not worth the time watching but that’s my opinion.

Why have we actors who can’t seem to project their voices or is the producer/director trying to install atmosphere and missing the point entirely if the dialogue is incoherent.

Apologies for my daily rant, perhaps the above is the reason my TV gets so little use.

@nuvistor Mumbling actors was a common complaint when I used to be able to read the audience logs. This is down to directors and actors seeking realism and it probably sounds fine in an edit suite with several thousand pounds of loudspeakers in front of you; not so much at home, where the sound quality of domestic TVs is poor compared to the past.

Hi Nuvistor/All, The BBC did a version of Treasure Island 3 or 4 years ago, maybe more, in Dolby Surround Sound. I set about to watch and listen to this and before I knew it I was sat in front of the TV speakers trying to hear what was being said. The scene was in a horse carriage. It was the same again in the rooms of the Inn. The dialogue was completely drowned out by the effects tracks. There were many complaints about this in the press. I can guess what happened there. It would have been monitored in the control room in full surround sound speakers so the centre speaker carrying the speech would have been clearer. The trouble with this is your TV mixes these tracks down to stereo using the parameters you set for the Dolby Decoder to mix down to stereo as most viewers would be listening on the sets stereo speakers only. This mix in the first place was terrible as I was listening in full surround sound and I was still struggling. I think they had to re-mix this in the end. When mixing surround sound there is a monitor button so the sound engineer can listen to the stereo mixdown track to check it is ok. All Sky's sound engineers check this frequently. Dolby surround sound is quite a process as you declare your channel mode 3X2L 5.1 (this is 5 channel surround plus low frequency effects) then the Dialogue level to start with, normally -23dB and you then mix your commentary or spoken tracks to this level. You then mix your surround tracks to your wanted level. There are parameters set to for your Dolby Digital decoder in your TV to mix downmix to stereo. Dependent on the content the surround channels will be attenuated by -3dB, in Sport it will be -6dB, surround channels can be higher in a football stadium, you can imagine why. The surround sound in the TV station is carried around in Dolby E, this being classed as almost lossless but also had a guard band during the vertical interval so it can be switched live without a problem. This is then converted to Dolby D for home delivery, more compressed but also a continuous stream so cannot be switched without causing an almighty glitch, Dolby D is therefore encoded on the output of the Transmission chain.

There was a setting on Dolby's original Dolby E encoders where you set channel mode, the dialogue and mixdown levels amongst others and there was a setting said "do you want these parameters to be forwarded on"!! As these where for use by the consumer decoders I used to think why is this setting there!!. Of course it has to be passed on so make sure it is marked to do this. The BBC fell foul of this setting big time during the Eurovision song contest one evening. You could only just hear the singers. The pass on metadata box was left unchecked so your decoder at home had no data so went to single channel mode playing out the front left channel of the 5 only. You can imagine what happened. I made a mistake when we built our first HD channels. I was testing channel levels and had removed the centre channel audio BNC this carries the speech so the movie went out almost speechless! Not many people had HD sets so nobody picked it up except the channel owner, we all make mistakes.

Below I have added the Dolby Metadata field so you can see what we have to set.

Oh how I wish for Mono TV

@hurty We all make mistakes, that’s ok.

I just feel that mistakes aside, if the dialogue is incoherent then sound effects are worth nothing.

Looking at the display of setting that need attention from the photo you posted makes me think is it not too complicated.

Poor sound as been a contention for many years even in the 405 era but it just seems that the more changes that can be made, producers/directors will try to use them and not appreciate the mess that can be made.

Just to show how well it can be done, I am now listening to Politics Live on BBC, dialogue easy to listen, clear and well presented, excellent. 

Hi All, I watched Politics Live today and yes it was nice and clear but it is a straight forward program to do. Most news programs just put the commentary down the centre of the stereo image. The mic's will be mono.

It is funny how audio in television has moved over the time. I have been involved in most of this as started with mono audio with 625 line colour. Worked in Telecine first then VTR. Feature films used to be provided with stereo at the start of this in the 80's but as was the case then broadcast in mono. It was when I was at Thames in the 80's onward that stereo was being looked at but no method of transmission. This country was not going to go down the dual sound carrier route. It was Nicam brought stereo transmission.

As an aside when I worked in Telecine (BBC) Widescreen or Cinemascope films where always shown full height 4X3. The powers that be said the public would not accept letterbox presentation, they won't like the black bar at the top and bottom of the picture. This meant in Cinemascope you only had 1/2 the picture, if two people were talking to each other you had to pan to the one talking then pan across to the other person as he/she spoke. You lost the entire concept of what was there. It was much the same with the many formats of Widescreen. In the rest of Europe the films where being shown in their original format letterboxed, no problem. You my struggle a bit with a Sinclair Microvision set! When Widescreen Television came out I belonged to a Ham radio Club and most people were saying what is the point, I won't be bothering with that. I was working on Widescreen and work had a Philips Widescreen 32" TV, one of the first sets to be released. The club arranged for me to do a demonstration of Widescreen TV, so said TV, Sony BVW65 tape player and a Widescreen recording of Lawrence of Arabia arrive to do the demonstration. I started with the desert scenes which in 4X3 don't look at all interesting but in 16X9 you see the marvellous desert backdrop come to life. There wasn't a peep herd from anyone and I sigh of disappointment when I had to stop the playback. Not one complaint of widescreen was herd after that. How it changed the perspective of the film! Sky went full into 16X9 widescreen presentation. Typical BBC no! they went for 14X9, a major mistake which they where then stuck with, no help for Widescreen television manufactures there. Cathovisor may have something to say about that😉. 

Back to the audio then. So television went stereo with some content. If I pick movies as the main point. The movie makers would mix the stereo to what you see on the screen so someone on the right came mostly from your right speaker and someone on the left would come mainly from the left speaker. The audio is diverged to match what you are looking at. Just to say this must not be over done! When done correctly it worked very well.

When stereo came out in audio I wasn't that impressed. But one day I fixed a Philips Stereo Record player with removable speakers and it was a Music For Pleasure record I was playing and the stereo image was perfect. each instrument in its position across the stereo image, I was impressed when stereo was mixed correctly it worked so well.

This also worked well with stereo movies, you could make a telephone ring in another room, I've seen people get up to answer their phone before now. We have two ears so if you know how to mix audio well and keep an eye on phase and also mic it well then you get good results.

Next comes surround sound 5.1 with your 6 speakers. Front Left, Centre, Front Right, Rear Left, Rear Right and Low Frequency Effects. Now the movie makers and some TV productions put all the voice on the centre channel only. To me this has been a major step back. It just does not create the same as you got with Stereo. This is my personal experience of audio.

The last 6-7 years I've been working with Dolby Atmos productions which contains object audio assets, not much use with Dolby Digital Plus as you have no metadata to take any control but with Dolby AC4 you could deliver the commentary as an object and give the customer control of this object so they can set the commentary level themselves, one of the biggest complaints we get is "The commentary is to loud" and "The commentary is to quiet" on the same match, you cannot win. With Atmos audio you get at the moment in TV 4 X height objects so your sound field now has height added. That does work. You notice this as when you got back to a 5.1 presentation you notice the audio field go as flat as a pancake. You will now need your 6 surround sound speakers plus four height speakers one in each corner of your room at ceiling level.

You can go out and buy your new TV which will do Dolby Atmos! yes it will but it will be a downmix to stereo, so make sure one, that it has been mixed correctly and two, this matches the provided downmix metadata, fun all this, so much more to go wrong.

Again hope this is of interest to someone and I hope people appreciate what goes on to make audio correct behind the scenes. Long live the PPM Audio was always correct using these.

@hurty Yes it’s very interesting reading.

For the made for cinema films I appreciate the difficulty in trying squeeze what the cinema audience expect and what the average TV in most homes can deliver. However content made for television need not suffer that problem and I feel the producer/ director has that many choices and little what seems expertise in choosing them leaving the poor sound technician to sort it out.

It could be folklore but the Jamaica Inn programme a few years ago had such poor sound that the directors wife asked him to turn the subtitles on while they watched it at home.

It could be I am being too harsh and just a grumpy old man.👴 

Anyway has I said it is interesting seeing the problems in transmission.

Hi Nuvistor. That was the program, Jamaica Inn. I can only say it was complete incompetence on that production and a lack of any experience of doing surround sound mixing coupled with improper monitoring. All the surround channels were mixed at the dialogue normalisation level. I would use that production as an example of how wrong you can get your multichannel audio. I don't believe they ever monitored the mix-down. The full channel surround wasn't done correctly either. It maybe someone turned the level up on the centre speaker. You are supposed to calibrate the speaker levels before you start. I was at a loss trying to think what happed there. You are not being harsh about Jamaica Inn it was just so wrong. It has gone down in history that one.

Hello All, Left the subject of PPM's behind a bit there but hopefully the subject of Television Audio level complexity is appreciated. Back to the humble PPM (Peak Program Meter). It first appeared in 1932 and was Valve Driven. Its present form now has been used since 1938 and still in use today but maybe in an LED bar-graph form. In my opinion not as nice to use. Below is a picture of the PPM scale.

The reference level is PPM 4 which equals 0dBu 775mV RMS. Each division between 1 and 7 is 4dB. Above and below this is non linear. If the meter was sat at PPM5 then your signal is +4dBu. It you were watching/adjusting the audio level for a talk program like News you would be aiming to peak the speech at PPM5 to 5 1/4 and allowing it to sink to PPM2. This gives a nice listening level and dynamic range for speech. PPM6 is the maximum allowed. If you went over this in television it would lead over deviation of the FM intercarrier sound, distortion and on AM radio you could collapse the carrier to zero and the transmitter would not be at all happy. There was protection for this at the transmitter sites but if you did go over a few seconds later the "Red" phone in the control room would be ringing with a telling off. The decay time of the PPM is about 2-3 seconds.

Pop radio stations use an "Audio Compressor" to squeeze the audio dynamic range to a couple of dB so your PPM would be sat at just below PPM6. Makes listening in a car with high background noise much easier. Radio 3 was a bit of a challenge.

When I started in Broadcast Television and met this stuff I learned about Balanced Audio connections. You had Jackfields where the jacks look like standard 1/4" jacks but where different as they where Post Office Jacks with a different profile, having a slightly smaller ring diameter. So the same Tip, Ring, Sleeve connection but carrying balanced audio across the ring and tip. Not stereo capable, you would need two for that. Post Office jacks are very little used these days. All your vintage TVs would have been feed via these connections. I would say P.O. jacks have disappeared in the last 25 years. Equipment mounted in bays would be connected by XLR connectors which I mentioned earlier.

Below are Male/Female XLR connectors and a standard 1/4 inch jack verses a P.O. jack.

Any questions feel free to ask.

Posted by: @hurty

↑

and a standard 1/4 inch jack verses a P.O. jack.

Or in very old BBC-Speak: Igranic versus Post Office jacks. Later, A and B gauge.