What is 0 Level??

[AndrewT]

Hi,
I have recently installed both a QU16 and QU24 in two small OB TV trucks.
Can anyone tell me what A&H regard as O Level either as a true voltage or PPM level biased BBC?
In trying to calibrate the audio chains in both trucks I notice that the O/P level on the single Mono outputs seems higher than those joined L/R Stereo outputs?
Any thoughts!
Cheers,
A

[timhum]

Zero Level as defined as a reference level, for example if you are sending zero level tone as a reference is 18 dB below peak level and is what we line up to in TV land, there is sometimes, on professional equipment, an indication at that level on a meter. It corresponds to PPM 4 on an old fashioned analogue meter.
To be pedantic, it should be -18dB in an EBU system and -20dB in a SMPTE one but what is 2 dB between friends!

Using this level gives 10 extra dB of useable level before nasty digital overload distortion kicks in if you compare it to the old analogue PPM meter. The 10 dB safety margin is useful because it makes any very short spike which does not show on the meter, pass safely through undistorted.

Post production may gripe that they have to turn up their speaker volume, iI have had that and on that show I put a limiter in the last 6dB and gave them a hotter mix which they seem happier with.
On other shows where the sound is remixed in the dub from ISO tracks, the same post production bods like to say that “yellow is the new red”. That stops any possibility of overloading and they can add noiseless gain of course. There, the professional standard of peaking at -10 dB with a “Zero Level” reference at -18dB would apply.

I found this on Wikipedia, it says what I say above with references.
Because quasi-peak PPMs indicate neither loudness nor true peaks but something between the two, it is important to allow sufficient headroom when using them in the control of digital audio levels. The EBU convention (R68) provides for this by defining Alignment Level as −18 dBFS.[22] Thus a peak to the Permitted Maximum Level as indicated on a quasi-PPM corresponds to −9 or −10 dBFS. This 9-10 dB margin allows for operator error, the true peak typically being several dB higher than the PPM indication, and that subsequent signal processing (e.g., sample rate conversion) may increase the amplitude.

SMPTE RP 0155 recommends a different alignment level, corresponding to 0 VU, of −20 dBFS.[23] The two conventions result in line-up tone levels that differ by 2 dB, but in practice the level of programme modulation tends to be similar.

The SMPTE and the EBU agree that regardless of whether −18 or −20 dBFS is used as the Alignment Level, that level should be declared and that in both cases programme should peak to a Permitted Maximum Level of −9 dBFS when measured on an IEC 60268-10 quasi-PPM with an integration time of 10 milliseconds.[24]

So your question is a good one which has at least two answers depending on your customer. It might be best to talk to your post sound people to see what they would like or to tell them what they are going to get so everyone knows where they stand. I last sent zero level tone to a camera about 10 years ago, even professional ones don’t always have a calibrated meter on them but if you don’t mod over -10 dB you will keep your job!

[AndrewT]

Hi,

Thanks for your reply.

I have been working to PPM Level 4 or as one engineer told me PPM 4 and a needle width!?

My understanding has always been that if you take 775mV to a 600ohm load you could refer to that as ‘Zero’.

If I remember correctly some mixer manufacturers work to 775mV and others work to 1V.

Mind you in a mixed analogue audio / digital video truck attempting to reconcile encoded audio levels on an SDI stream to the original analogue can input make for interesting reading – especially on Blackmagic Design Equipment

Thanks again,

AndrewT

[timhum]

The definition of Zero Level is indeed 775mV into a 600 Ohm Load. The issue is that things have moved on since that specification was introduced and although it does remain a “standard” it used to refer to 8dB below peak level, now it refers to 18dB below peak in order to contain short peaks which would distort in digital systems. When the PPM system was introduced back in the 1930’s, system noise was a big issue and keeping programme material above the hiss was important. Now microphones and audio circuitary are commendably quiet, the issue is more with the bubbling mud type of peak distortion you get with digital systems, hence the “standard” shifting to -18dB from the now historic -8dB. It does not help the situation when some cameras used for broadcast use do not have calibrated meters and editors complaining they have to turn up their speakers.
As regards the QU 16 I have and just looked at, if you ignore the red peak LED, the next one down is +12. If you consider that to be the peak, then your Zero Level calibration tone should be sent at -6db (i.e. 18dB below +12) and helpfully there is a green LED with -6 next to it!!! So with only a decibel or so error you should keep in the green with only a very occasional bottom yellow LED lighting up which is as good as you need to get on the hoof with un-rehearsed live action happening on your OBs.
Thank you for your question, I have been working with the QU16 mainly as a PA mixer so it was good to check how it would integrate into a broadcast standard setup. It also occurs to me that the post production sound guys who say “Yellow is the new Red” know their stuff and have a handy catchphrase to pass on their knowledge!

[AndrewT]

Once again many thanks for your input. My early experience of ‘live’ radio OB’s goes back to constant impedance Rotary and Quadrant faders! – None of this VCA rubbish!!!
Cheers
AndrewT

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