PMW-200 -- any tips / advice on how to correctly use "AGC Spec" & "Limiter" settings?

[John M. Kim]

The Sony PMW-200 manual is a bit cryptic (for novices, at least) when it comes to the audio's automatic gain control and limiter settings and I could use some help.

The manual merely states switchable settings like so:

AGC Spec
–6 db / –9 db / –12 db / –15 db / –17 db
(Select the audio input level AGC.)

Limiter Mode
Off / –6 db / –9 db / –12 db / –15 db / –17 db
Select whether to activate the limiter when a loud signal is input while the AUDIO SELECT switch is set to MANU (manual).

I'm shooting mostly documentary and event-type footage using a Rode NTG-3 with a rated sensitivity of -30dB ± 2dB.

In my previous experience with DSLRs, I've treated AGC as "bad, bad" because of the way it artificially boosts quiet sounds. Does this translate into the world of the PMW-200 as well? Should I leave the AGC at -17db?

And in a related question, does the Limiter setting refer to the amount a crashing, clipping sound will be lowered in dB? Or does it indicate the level past which a limiter will kick in and lower the volume level?

Sorry if these questions are basic. I searched for a suitable answer to these questions but came up blank.

[Paul Gallagher]

I would like hear some answers on this as well, I own the PMW150, the UK version of the PMW160 and the audio on these is the same as your PMW200, but its way over my head all the extra settings.
This thread might also help answer some of these questions.
http://www.dvinfo.net/forum/sony-xdc...ml#post1816414

[Roger Gunkel]

I'm not familiar with the equipment and software that you are using, but from a technical point of view there are different ways in which limiters and Agc work.

Limiter - you set your audio up manually to whatever level you expect to record at, doing a test recording to get it right. Then set the limiter to the point at which you want the maximum level to occur. Any level above this will be squashed to the limiter settings. If the variations in audio level are not huge, then you can set your minimum levels high enough to avoid system noise, knowing that sudden transients will be limited rather than overloading. Setting your input level too high will result in too much squashing and not enough headroom for dynamic range. Think of it as a rubber ceiling, a tall guy (high input volume) jumping up and down will hit it more often than a short guy (low input volume) Squashing or limiting the sound too heavily, results in changes to the sound, as a sound waveform is made of many different frequencies. Heavy limiting will squash the loudest frequencies down to the level of the lowest, with sometimes drastic sound differences. It could end up sounding like someone jiggling the sliders on a graphic equaliser, so use limiting with care.

AGC - is adjusting the overall gain within a set range, so if the volume is low, it is automatically brought up to the ceiling threshold. As the volume is increased, the gain is reduced to keep it within the range.

In many ways AGC works from the opposite end to limiting. AGC brings the quiet sound up to the ceiling level, including any system noise, while limiting keeps the set basic levels the same but stops the loud sound from going through the ceiling.

Hope that's not too confusing.

Roger

[Paul Gallagher]

Thanks Roger, that does make sense. Its with wedding bands my issue is, their songs vary that much I dont feel that the limiter mode will be of any help. My last one I had the AGC turned down to -12db, this stopped a lot of clipping but gave me a very flat dull waveform to work with and it still clipped on higher base sounds.

[Alister Chapman]

AGC is for auto level control. The limiter is for when your using manual audio control and the sound levels become excessive to prevent distortion by limiting how high the audio will go. I usually set the limiter to -6db so that it will kick in on any audio peaks that are 6db below clipping or higher (about two blocks below the red bars). AT -12db the audio level is kept lower still and will restrict your recording range requiring you to record at a lower overall level.

[Paul Gallagher]

So for recording a live wedding band Alister would you use the on board mic or a phantom XLR one and monitor it on manual levels with the limiter at -6dB?
I had tried the on board on auto levels with the AGC at -12db, but it still clipped and I wasn't happy with the overall sound. I used my Z1 for five years on auto levels and it had gave me superb sounds in this environment.

[Alister Chapman]

AGC at -12db means the audio will never go above -12db, so that's really restricting your recording range. For Auto, I'd just leave the AGC at -6db, the extra 6db means you have double the range to play with and your still 6db below clipping. If the audio is still distorting then you may need to reduce the input gain a bit by reducing the EXT MIC Ref level.

To set the EXT MIC REF level I set the audio control to manual, set the audio pots to the mid position and speak at a normal level about 6" from the mic and then select the EXT MIC REF level so that the audio peaks are closest to half way up the audio scale. This put the audio gain in the right ball park for that particular microphone.

[Roger Gunkel]

Originally Posted by Alister Chapman

AGC at -12db means the audio will never go above -12db, so that's really restricting your recording range. For Auto, I'd just leave the AGC at -6db, the extra 6db means you have double the range to play with and your still 6db below clipping. If the audio is still distorting then you may need to reduce the input gain a bit by reducing the EXT MIC Ref level.

It's been a while since I have delved into the figures, but I believe altering the AGC from -12db to -6db will actually make a 4x variation in the ratio. 3db gain variation doubles or halves the gain depending whether it is + or - so in practical terms, a 6db gain increase will quadruple the apparent volume, or base volume level.

Roger

[Paul Gallagher]

Originally Posted by Alister Chapman

AGC at -12db means the audio will never go above -12db, so that's really restricting your recording range. For Auto, I'd just leave the AGC at -6db, the extra 6db means you have double the range to play with and your still 6db below clipping. If the audio is still distorting then you may need to reduce the input gain a bit by reducing the EXT MIC Ref level.

To set the EXT MIC REF level I set the audio control to manual, set the audio pots to the mid position and speak at a normal level about 6" from the mic and then select the EXT MIC REF level so that the audio peaks are closest to half way up the audio scale. This put the audio gain in the right ball park for that particular microphone.

Thanks Alister, I'll give this a try.
At saturdays wedding I just used a EX1R I also bought recently and it has only 2 AGC settings, high and low, I set it on low and the sound from it at that setting was perfect.
Next wedding I'll run the 2 side by side with these new settings on the PMW150 and compare them.
If its still not right I'll just use the EX1R for my dancing as its better in the low light anyway and I'm still not confident in the PMW-150 in this sound environment as of yet.

[Alister Chapman]

Ahh the dreaded decibel confusion, not helped by use of the generic dB instead of dBm, dBu, dBFS etc.

6db when referred to a change in voltage (or camera gain) is double/half the voltage, this is dBu. Assuming the meters are dBFS (Decibels relative to Full Scale) then -6db is a halving of the recording level. Audio recording normally uses dBu or for digital recording dBFS.

3db would be a doubling in power, but we are not measuring power, we are measuring voltage.


From Wikipedia (I had to double check as even though I studied electrical engineering at university this still trips me up):

A change in power by a factor of 10 is a 10 dB change in level. A change in power by a factor of two is approximately a 3 dB change. A change in voltage by a factor of 10 is equivalent to a change in power by a factor of 100 and is thus a 20 dB change. A change in voltage ratio by a factor of two is approximately a 6 dB change.

[Roger Gunkel]

So what you are saying Alister is that a 3db increase in a p.a. amp output would double the listening volume, but a 6db increase in input gain would be necessary to double the input level?

Electrical engineering maths is not my thing I'm afraid, so the clarification will be enlightening (or confusing).

Roger

[Alister Chapman]

Yes that's correct.

db is a ratio, not an absolute value, so +6db gain is 6db more than the original value, but does not specify the original value.
There are two different formula used to calculate db, one for power such as energy density, acoustic intensity and luminous intensity. Then there is a different formula for field intensity, things like voltage amplitude, current, velocity, pressure. In electrical systems the db power is proportional to the square of the db field.