SD302 1kHz test tone is at -20 dBFS (0 dB VU) level as defaul, just like it should be.

It is also possible to get a full level test tone out ot the SD302 via the menus, which is at 0 dBFS (+20 dB VU). This can be used to test the system match, that is, set the level excatly so that levels are maximized but recorder clipping is impossible.

Unless your in the U.K. in which case -18 = 0db analogue (ppm4) and we always aim to peak at +8 db analogue which is -10dbfs. 8 db above tone.

The U.K. method makes more sense I think actually as it refers directly back to the old practice and is a logical progression.

Oh while I'm at it here in the U.K. broadcast world we have pretty much avoided V.U.s completely and use PPM's which actually makes it a lot simpler.

Does this mean that broadcast cams (varicam, digibeta, etc) record 24 bit audio, since their meters indicate that peaks should be at -20? (this is opposed to cams like the XL2, DVX, etc. where the peaks want to be at -12) .

They're actually the same. (Huh? 0 = -20???? --- YES!) You're getting digital and analog mixed up. The meters on the Sound Devices 302 are analog meters, selectable for either VU or Peak reading, that are calibrated so that 0dBVU on the meter equals an output signal voltage of 0dBu, 0.775 volts. When that signal is input to the DV camera or a digital recorder, it should line up to -20dBFS on the recording device's digital reading meters. So 0dB on the SD302 IS -20dB on the recorder or camera. Same signal level but different meter calibrations.

Steve, Petri - thans for the comebacks.

Let me try to see if I've got it.

The 302 manual says that tone level can be set to -20dBu, -10, etc and that 0dBu is the default

Their recommended calibration procedure is to generate 0dBFS and set this to just below clipping on the recorder.

I think this is where I was getting confused between the 0dBFS and 0dBu - I was mentally not paying attention to the distinction between dBu and dBFS when I saw the comment in the manual about 0dBu being the default and I was reading it as 0dBFS.

Anyhow, I calibrated my recorder per the instructions to use the 0dBFS tone and set it at just under clipping on the recorder, and when I sent the default 0dBu tone from the mixer, the recorder indeed read -20dBFS as it should.

I think the reason that they recommend the "just under clipping at 0dBFS" method is that some recorders don't have exactly the same scale as the 302, and even if 0dBU on the mixer reads -20dBFS on the recorder (which would be an easy way to set things up) you might still get clipping on the recorder before the 302's limiters kicked in.

Have I got it?

Jim, you got it. Another way to set the recording levels for maximum dynamic range is to send a full maximum level tone from the 302 and set it to just below full-scale (0dBFS) on the recorder. That will be about +20dBU actual signal level, so when you send a tone that reads 0dBVU on the 302 meters, it will read about -20dBFS on the recorder meters and the 302's limiters will kick in just before the recorder clips.

No his explanation was confusing. 0dB tone from the mixer to -20 at the camera inputs.

Mixer output and camera input both must be set to line or mic, but not one to mic and the other to line.

Regards,

Ty Ford

It is confusing and I usually am a pretty good writer/explainer. How's this....

1: Turning on tone on the SD302 (or 442) that reads 0VU on the mixer's meters means the mixer is putting out a 0.775v signal level on its outputs.

2: According to the standard, a camera, recorder, or workstation that's recording digitally should have its recording level controls adjusted so that a tone with a level of 0.775v at its inputs will result in its meter reading -20dBFS.

3: Real world signals ain't simple tones and meters react differently to tones and complex signals. Setting tone that averages 0.775v to read -20dBFS on the recorder and then switching to voice that also averages 0.775v without changing any settings will actually result in the recorder's meter reading closer to -12dBFS with the same setup.

Your 3: is still confusing to me Steve. I know what you're trying to say, but if I put on my novice brain, I don't understand.

Lets see......Tone doesn't have the peaks a human voice or piece of music does. When you align tone at -20 on the camera, you should also check your metering on the mixer to make sure it is set to a Peak reading mode.

I ran into this issue with a friend recently. He had most of the calibration right but was concerned that the readings were so low. He wasn't setting the mixer to read peak. After I showed him how to do that, he saw the correlation between the mixer metering and the camera metering and the concept "clicked" in his head.

The other most frequently missed problem is that you have to have both mixer output and camera input set to mic or to line. THEY HAVE TO BE SET THE SAME ON EACH DEVICE. Mic to mic or line to line.

Regards,

Ty Ford

Peak reading mode
 

How is this done? Is it possible to explain this here in writing? If it's different for each mixer, then let's use an SD 442 for example.

-Chris

With the SD442 there's a meter mode pushbutton control on the front panel. Each push switches to the next of 4 possible modes and then after the 4th one, back to the first to repeat the cycle all over again. It starts out in VU meter mode, then a button push switches it to Peak Reading, then to a combination VU/Peak mode where the VU is a continuous series of LEDs illuminated ending at the VU level followed by a sequence of several dark and then a single LED showing the Peak, then finally to a Peak Hold which shows the same reading as the Peak meter but holding the indication for a few seconds before decaying to give you time to actually see the sucker. You can easily see on voice the difference in the VU and Peak on the same signal by setting the mode to VU and speaking into a microphone while adjusting the fader until the meters read 0VU and then pushing the button to switch to Peak mode while continuing to speak taking care to keep your voice the same. The meter immediately jumps up to read + 6 to +8 dB higher and it actually would be higher than that but the real peaks are occuring too fast for your eyes to see the LEDs illuminate. Or send it some music while in VU/Peak mode and see how far above the bar indicating VU the peak single LED will land. Another interesting demonstration is to use a tone plug to send tone to an input, use the fader to set it to 0VU level, and then switch the metering between the modes. Unlike voice that reads signifigantly higher in Peak mode than it does in VU mode at a given perceived identical volume level, the steady sine wave reads exactly the same in both modes.


(Have a new 442 and I've been running those experiments myself just to get gut-level familiar with how the meters respond in different situations.)

The trick with recording is that even a single cycle transient, far too short for any meter to respond to, that goes above 0dBFS (a level represented digitally by 1111111111111111 if recording 16bit) will clip with potentially audible distortion. So you want to set the average signal levels low enough that under no circumstances will the waveform EVER rise that far yet high enough that the highest peaks just almost reach it so as to realize the best signal to noise ratio while preserving the full dynamic range of the original performance.

The SD 302 has the same button pushing method to switch among the metering modes.

thanks steve, much more simple than I thought.