Measuring audio noise levels with and without ALC

[Tramm Hudson]

Originally Posted by Jon Fairhurst

For anybody recording into the 5D from an external source, apply EQ with a low frequency shelf at -Inf. That should calm your meters so they more accurately reflect the noise of the system.

The AK4646 has a digital 5-band EQ that might be usable for this -- the frequency, bandwidth and gain are all selectable for each of the bands. I believe they are all initialized to 0 dB gain and disabled by default.

I might not be setting the HPF and LPF correctly, either. My code was writing all zeros to the coefficients, but it looks like the correct thing to do is to disable the HPF, LPF, FIL3, EQ and GN bits in the digital filter select register (0x11). We may need to make some more trials this weekend to see if we're missing something.

[Chris Barcellos]

Originally Posted by Jon Fairhurst

With the noise plots, there are four areas:
1) DC/Low Frequencies: Both the pilot tone and manual gain tests have LF problems. This can be remedied in post. I can reduce the resting level of the meters by over 20 dB with a subsonic filter.

2) Pass band: The pilot tone method has noise throughout. The gain reduction method has almost none. The difference is 20 dB or more.

3) High Frequencies: On the pilot tone method, you can see the noise drop to almost nothing somewhere around 16 kHz. I presume that this is the anti-aliasing filter. It's hard to see that drop off with the manual gain method, as the noise is already near that level.

4) The pilot tone: Yes, there's some bleed at the frequency of choice. That doesn't apply with the fixed gain situation.

For anybody recording into the 5D from an external source, apply EQ with a low frequency shelf at -Inf. That should calm your meters so they more accurately reflect the noise of the system.

Jon:

Are you saying the camera filters out any thing at 16 kz and above, so with my tone at that arbitrary 15,400 that I had originally landed on, if I push it up to about 16,000 or a bit more, I might lose what I had been working at the somewhat lower frequency ?

[Chris Barcellos]

Originally Posted by Tramm Hudson

The AK4646 has a digital 5-band EQ that might be usable for this -- the frequency, bandwidth and gain are all selectable for each of the bands. I believe they are all initialized to 0 dB gain and disabled by default.

I might not be setting the HPF and LPF correctly, either. My code was writing all zeros to the coefficients, but it looks like the correct thing to do is to disable the HPF, LPF, FIL3, EQ and GN bits in the digital filter select register (0x11). We may need to make some more trials this weekend to see if we're missing something.

As Arte Johnson said on Laugh-In, very interesting.

[Jon Fairhurst]

Originally Posted by Chris Barcellos

Are you saying the camera filters out any thing at 16 kz and above, so with my tone at that arbitrary 15,400 that I had originally landed on, if I push it up to about 16,000 or a bit more, I might lose what I had been working at the somewhat lower frequency ?

Hi Chris,

I tried a 16kHz tone, and I had to really crank the levels to push the ALC down. If the pilot tone is cut out by the filter, it won't drive the levels down at all.

I'm thinking that a mid frequency tone is best. It will have less crosstalk, and it will definitely not get cut off by the LPF.

Such an approach might make the 5D okay for audio, but Magic Lantern can definitely push the noise down further. Of course, in either case, with the gain pushed down, a hot signal - either from a hot mic or good active preamp - is needed.

[Jon Fairhurst]

Originally Posted by Tramm Hudson

The AK4646 has a digital 5-band EQ that might be usable for this -- the frequency, bandwidth and gain are all selectable for each of the bands. I believe they are all initialized to 0 dB gain and disabled by default.

If it's quick, you might try -Inf for the gain and 20 Hz or lower for the frequency with a bandwidth that would include DC.

That said, this is a minor irritant that can be handled in post. I'd put it way low on the priority list.

I might not be setting the HPF and LPF correctly, either. My code was writing all zeros to the coefficients, but it looks like the correct thing to do is to disable the HPF, LPF, FIL3, EQ and GN bits in the digital filter select register (0x11). We may need to make some more trials this weekend to see if we're missing something.

I would definitely enable the LPF. Assuming that's what I'm seeing in the noise plots for the standard firmware (16kHz cutoff), I can easily verify if it's enabled or not.

[Jon Fairhurst]

I had time for one quick test tonight. The LPF is not currently enabled - there's no 16 kHz cutoff.

Gotta run...

[Chris Barcellos]

Okay, I admit I am no sound tech. School me.

LPF , low pass filter. only lets lower frequezies than setting pass, and hpf (high pass) only lets higher frequencies pass than setting. Have I got that right ?

And does this mean 16khz is pushing the envelope of the system in general ?

[Jon Fairhurst]

Hi Chris,

A LPF is needed before the analog to digital conversion. The sampling rate on the 5D2 is 44.1 kHz. Such a digital system can't carry any information beyond 22.05 kHz. If you provide an input frequency above that (the Nyquist frequency), it will alias.

A great example of aliasing is when helicopter blades look like they're going backwards. Any frequency beyond the Nyquist limit gets reflected down into the audible frequencies - an input near 44 kHz could sound like a low frequency rumble!

The HPF, on the other hand, is for removing rumble due to handling noise and wind buffeting.

[Bill Davis]

Originally Posted by Tramm Hudson

The AK4646 has a digital 5-band EQ that might be usable for this -- the frequency, bandwidth and gain are all selectable for each of the bands. I believe they are all initialized to 0 dB gain and disabled by default.

I might not be setting the HPF and LPF correctly, either. My code was writing all zeros to the coefficients, but it looks like the correct thing to do is to disable the HPF, LPF, FIL3, EQ and GN bits in the digital filter select register (0x11). We may need to make some more trials this weekend to see if we're missing something.

Tramm,

When you get a Beta version done, I've got a custom built VO booth that's about as close to an anechoic chamber as you'll get outside of an audio lab - along with a Neumann TLM 103 with a 70-80 db self noise rating that could provide a good "real world" test.

That signal chain fed into my Mk5 d ii should be dead quiet if you can actually defeat the dreaded AGC via code.

Plus I can easily videotape the test for you for posting the results on-line.

So let me know if I can help. (The biggest advantage of owning this studio is that I don't have to charge myself to use it!)

This would be great for all of us - although it would likely mean a huge flood of used H4n's on ebay!

Good luck with your development efforts!

[Matthew Roddy]

Originally Posted by Bill Davis

Tramm,

(The biggest advantage of owning this studio is that I don't have to charge myself to use it!)

I'd say you charge yourself to use it and make a small fortune.
...

I'm looking forward to the test results.

[Tramm Hudson]

Originally Posted by Jon Fairhurst

The LPF is not currently enabled - there's no 16 kHz cutoff.

Is it on in the stock firmware? My recent beta was supposed to enable it, but I think that the bits in the digital filter select 1 register were not configured correctly.

If you have more time for testing this weekend, perhaps we can coordinate some more rapid fire testing to see if we can get these settings hammered out. I'm hoping that we can get these configured with reasonable defaults so that I can spend more time on figuring out how to create menus and read/write commands to the lens.

[Tramm Hudson]

Originally Posted by Jon Fairhurst

The LPF is not currently enabled - there's no 16 kHz cutoff.

From the spec sheet, the default parameters for the HPF as a wind-cut filter at 150 Hz @ 44.1 kHz (F1A=0x1FA9, F1B=0x20AD) and the LPF is not enabled by default.

Canon's windcut filter changes the parameters to F1A/B=0x1FB5 and 0x2096 for the HPF and the LPF at F2A/B = 0x0ED4 and 0x3DA9. Inverting the function on page 34 of the spec sheet is left as an exercise to the reader.

Do you have a suggested set of frequency cutoffs that should be programmed in for the tests?

[Jon Fairhurst]

Originally Posted by Tramm Hudson

Do you have a suggested set of frequency cutoffs that should be programmed in for the tests?

For the HPF, it would be nice to get rid of the DC offset. I'll download the spec sheet and do my best to calculate the best values available. Right now, though, I'm about to turn off the PCs and do some noise floor tests...