[Boatanchors] OT: Question about audio

[Brian Clarke]

Hello Eugene,

In audio, PPM means peak program meter. It is particularly used in recording 
where you don't want the peaks to exceed the 'ceiling' of the recording 
channel. PPMs are mostly found in professional audio equipment - additional 
processing electronics are required - not just the d'Arsonval meter head 
alone. A PPM would also be useful in amateur radio to ensure that you don't 
over-modulate and cause splatter. Typically, the meter needle moves upward 
very quickly and then hovers for a second or two so you can see the actual 
peak.

The Volume Units (VU) meter has very specific ballistics, ie, the way the 
meter needle moves with respect to the rate of change of Voltage fed to it. 
It is a passive, consumer-level device with nothing special to recommend it 
except its extreme cheapness and perhaps its ability to measure 
constant-levels of Voltage. VU meters cannot indicate fast-rising peaks, 
such as when people use plosives, eg,  b, p and t. Too many people rely on 
the VU meter on their audio equipment and the sound is highly distorted - 
because the peaks exceed what the channel can handle.

So, if you plan to measure your audio levels before feeding them to a 
modulator, I would recommend a PPM rather than a VU meter. If you follow 
this recommendation, then you don't need to know what the markings on a VU 
meter scale mean.

'Tone' was often used with Dolby recording equipment to establish that all 
elements in an audio chain could operate at the same level.

The LM 3100 series of LED drivers can approximate a PPM at much lower cost 
than a proper PPM with a d'Arsonval movement.

Cheers, Brian, VK2GCE.

On Saturday, February 15, 2014 2:45 PM, Eugene asked:


> Hello all,
>
> I am in the process of building a new shack. I am hoping to do more AM 
> than I used to, with more space for some nice sounding transmitters.  I 
> thought it would be sensible to have some VU meters in the shack that I 
> can route the audio to before heading to the exciters/transmitters.  The 
> idea is to make sure I am optimizing the audio levels going into the 
> exciters.  I plan to run my microphones through a pre-amp (Behringer Shark 
> DSP110) to boost the level to line level (probably targeting +4 dBu 
> balanced).  The transmitters I have will all accept "line level" audio at 
> various selectable expected input levels.
>
> The question is about VU meters. I have been looking on ebay and I've seen 
> some things I don't understand.    I have seen some VU meters that have 
> two types of meters on the same panel. One looks like the standard VU 
> meter showing -20 to 0 and then +1 +2 +3 in a red section.  The other 
> meter on the panel has the word "TONE" and "dB+Range" on it and the scale 
> is -12 to +12.
>
> Just some examples on ebay of these kinds of meters.
> 331098964167
> 231107605604
> 281193957524
>
> What is the meaning of these two types of meters? What do they mean 
> "TONE"?
>
> Question #2.  I have read about a term called "Ballistic" and the options 
> seem to be PPM or VU .  Is PPM something to do with peak vs. average? I 
> know typically in ssb I would set my wattmeter to peak and in AM or CW I 
> would set it to average. In an audio system, when would someone want to 
> look at peak vs average? (and is that what ppm/vu means?)
>
> Thanks very much for any input on this subject.
> 73 Eugene W2HX