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  1. #1

    How to align the audio meter?

    I have a couple of questions on audio meter that need clarification.

    First, what is the different between VU meter and PPM meter and its applications?

    Second, How can I align the two meter properly in order to read the audio content accurately on both meter? If possible, what is the recommended reference level and test level used commonly for broadcast applications.

    I would appreciate your help in clarifying the above questions. I am new to broadcast industry and need some guidance in setting this up correctly. Thanks so much in advance.

    Regards,

    Anthony

  2. #2
    Both meters are designed to take into account the fact that audio levels are dynamic - unless you're listening to a test tone, which nobody does.

    A slow average-reading meter is about useless, because it totally ignores fairly short-term high level moments. Also, since speech contains pauses between words and syllables, an average-reading meter will tend to have an unrealistically low reading on speech. In a speech and music program, this will cause the operator to turn up the speech mic too high.

    The VU meter was developed to overcome this. Its ballistic characteristics were calculated to give readings that correspond well with what we hear subjectively. If you put a continuous tone into a VU meter, it will reach 99% of its final reading within 0.3 seconds, and have no more than a 1% overshoot (if I remember right) before settling to its final reading. In other words, a VU meter is just a voltmeter with well-defined ballistic behavior; fast enough to "hear" the way the ear hears but not so fast as to oscillate madly.

    A traditional VU meter requires a 3.9K ohm resistor in series - without this resistor, its ballistic behavior will be much slower. With the series resistor, a VU meter reads "0" at +4dBu (which is also +4dBm if you're referencing to a 600-ohm circuit) which is about 1.23 volts. When broadcast engineers use the shorthand expression "plus four" this is what they're talking about. This often leads to confusion: the standard reference level, which the measurements are pegged to, is 0dBu which is about 0.78 volts. The standard actual level is +4dBu or 1.23 volts, both because that's how a VU meter is calibrated and also because broadcast engineers like hot levels. (The hotter a signal, the less sensitive it is to hum and other forms of crud.) For the same reason, sometimes a 4dB pad is inserted between a board output and VU meter, effectively reducing the sensitivity of the meter so it reads zero at +8dBu which is almost 2 volts.

    I say a "traditional" VU meter because manufacturers have gotten really sloppy about following the official specs. Often, a "VU meter" is nothing more than a generic cheap voltmeter with a VU scale stamped on it, neither calibrated to +4dBu nor anywhere close on ballistics. When you've got a studio with various meters in various places, you will have to make sine-wave adjustments to calibrate them all together. But from then on, it pays to know which meters you can trust when it comes to actual program material.

    By the way, if a VU meter and its resistor are hung directly across a 600-ohm line, it will create about half a percent of distortion. This was considered okay back when the VU standard was settled on. Modern outputs have an actual resistance much lower than 600 ohms, so there will be less distortion. Still, it is bad practice to hang a VU meter permanently across a line that people will actually be listening to. With opamps as cheap as they are now, any respectable console will have a separate amp to drive the meter. This often means the correlation between 0VU and +4dBu is lost. You'll have to check - it's no longer possible just to assume.

    So much for VU.

    Normal program material has peaks that do not show up on a VU meter. These are of a millisecond or even less duration, and are routinely 10dB above average, often more. If you are broadcasting, you cannot permit even momentary overmodulation. If you are doing digital recording, any momentary overload will create wretched-sounding aliasing. So you need a meter that will show you peaks that won't register on a VU meter and you can't hear. That's what a PPM (peak program meter) is for. A PPM typically catches peaks of 10 microseconds, and stretches them out long enough so you can see them. Some hold the highest peak until you reset it manually, so you can see what the highest peak in a whole program was.

    There is no "official" relationship between what a VU meter reads and what a PPM meter reads because there is no guaranteed peak-to-average ratio in actual program material. However, a sine-wave test tone ought to read the same on both. Once you have got them set up to read the same on a test signal, expect them to diverge widely when you get to feeding them actual program material.

    One more thing - since consumer equipment has improved to where it sounds like pro equipment, a lot of it is finding its way into studios. The standard "0" level in consumer equipment is about 0.3 volts, or about 12db lower than 0VU. So if you use it, expect to have to set the gain controls differently.

    Summary: the standard "average" broadcast level, 0VU, is +4dBu which is 1.23 volts. In a studio, levels are generally set so a 1.23 volt signal on a line will read 0VU on the associated meter, unless the calibration is set differently for some reason. A PPM, depending on type, should show the same reading on a test tone but will read perhaps 10dB higher on actual program material because it's showing you the peaks.
    Last edited by karl eilers; 27th Nov 2008 at 04:20.

  3. #3
    (Nothing to add. Once again I have run into the dreaded timeoutbot.)

  4. #4
    Thanks Karl for your quick reply.

    Just to double check with you. Both VU and PPM do not have the same response. Therefore, to make them equivalent using the same audio program, I have to send a line-up tone reading of 0VU on the VU meter and PPM should read at least 10 db lower than VU meter? There are some opinion that PPM must read 8dB lower than VU in order to align both meter read substantially the same with audio program. So which one to follow?

    Can I say that audio program should be adjusted to have peak amplitude of 0dB on a PPM in order to avoid any distortion.

    Thanks once again for your help.


    Anthony

  5. #5
    The correlation between meter reading and distortion threshold depends on the system setup. Most broadcast consoles can put out at least +24dBu - usually more - so you won't get distortion until the average level is 10dB over 0VU and the peak level is 20dB over 0 on the peak meter. Other parts of the signal chain may not have as much headroom. When a PPM is used to monitor transmitter modulation, it's set up so it registers its highest peak when the transmitter reaches 100%.

    In other words, there isn't a hard-and-fast answer. To complicate things, there are different kinds of PPM's. Some are calibrated in dB's and some just have a 1-to-7 scale. PPMs haven't been around as long as VU meters and things aren't as settled.

    If you want a PPM to read approximately the same as a VU meter, set it to read -8 to -10 on a 1.23-volt tone. Can you say that you'll stay out of distortion if you keep the PPM below zero? Dunno, depends on the system.

    A really good broadcast engineer takes the time to become familiar with every piece of equipment's standard operating level, clipping level, gain (gain range, if adjustable), noise floor, input impedance, output impedance (actual and intended load), whether balanced or unbalanced and if balanced, what type. Don't let me scare you; like riding a bike, this becomes second nature after a while.

  6. #6
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    What machine are you using? The Studer decks I've used have a very precise and pre-determined way to calibrate, depending on whether you are using VU or PPM metering. Furthermore, the Studer decks I'm familiar with can display either type of indication, and there is EXACTLY 6.0db of difference between the two, when displaying a sine wave test signal. So, for PPM meters, when playing test tape tone 1kHz at reference level you actually set repro adjustment pot to read -6dBm instead of 0dBm. Then adjust repro output control to provide operating level at tape outputs across a 600 Ohm load for NAB eq. (200 Ohms for CCIR/IEC1 eq). An operating level of +4dBm produces 1.23V at tape outputs. Now, don't touch anything, except to adjust meter amplifiers to read 0db. Your deck is now correctly calibrated for operating (line) level at reference fluxivity of your test tape, and meters are also correctly calibrated. You can switch back and forth between the two types of meters without needing to adjust anything. You can set one channel to display PPM and the other channel to display VU if you want to compare the two. If you later change to a different operating level you'll need to re-zero the meters at the corresponding new line voltage.
    Last edited by midimaniac; 28th Nov 2008 at 10:07. Reason: adding info

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