Clipped audio levels during capture with multiple hardware?
 

Several months ago I mentioned not being able to change the audio levels on my VC500 during capture. I've recently been looking closer at the audio, and I'm finding a lot of clipped audio in my captures. This happens with both my Diamond VC500 USB and Pinnacle 510-USB in Windows 10. I thought the 510 may be doing it because everyone recommends Windows XP instead of the Windows 10 I'm using, but the VC500 has current official Win10 drivers. I decided to try my Blackmagic Shuttle Thunderbolt, and also get clipped audio capturing with it under both Windows and MacOS even with its gain turned down. Same result capturing on my laptop. Same result with both JVC C101US and HR-S7800U VCRs. Same result whether I'm passing the audio thru a Pioneer ES15 or not.

On a whim, I decided to see if the ES15 DVD recorder still recorded, so I burned a DVD straight from VCR-->ES15. After ripping the disk, I was amazed to see no clipping!

Is there something about the audio signal that the VCRs output to a television that is too strong for the capture devices but that the ES15 knows to change before recording? What are you all seeing with the audio you are capturing?

Top waveform is the ripped DVD recording, bottom is the captured file.

Erich

I've encountered the same issue with capture cards. The VC500, Blackmagic Intensity, and empia cards all have the gain too high for what comes out of VHS decks for whatever reason. At least on the empia cards it's possible to turn the volume down with a registry setting, the blackmagic and VC500 windows drivers don't have any working way of doing this that I've found. Blackmagic card level settings, both for video and audio seem to apply after the signal is digitized last time I tried.

On our panasonic decks and 8mm cameras using the headphone out and turning down the volume a bit solves it, but otherwise one would need something external to lower the volume or use a different devce for audio capture.

Also noticed as you, that it doesn't seem to be an issue with DVD-Recorders. No audio issues whatsoever when capturing from my Sony DVDR, (or JVC one) with the blackmagic over HDMI.

I thought the pinnacle 510 had a bit more advanced drivers, maybe it has some audio setting somewhere?

Yes, it looks like the Blackmagic Shuttle Thunderbolt applies the slider settings after digitizing, which is why it doesn't help with clipping. It's too bad the ES15 only passes thru the audio instead if treating it as it does when it's writing the DVDs.

Looking thru the docs for the Mac version of the Diamond VC500 shows a gain control in their capture software, but I suspect it's the same kind of thing as the Blackmagic and is done after capture. Since I already have the Windows version getting the Mac version probably won't be any different.

I wouldn't mind getting a separate USB audio box if it provides high quality audio. Synching the audio to video afterward is easy. Anyone have a recommendation?

Erich

The Diamond VC500 Mac has completely different hardware to the black non-Mac one, the two ones we got here are empia-based, so it's possible to adjust the volume on them with the registry setting in windows. Wouldn't recommend them though, the newer ones are pretty mediocre as far as I know.

You can technically digitize the audio with the ES15, it, like most DVD-recorders has digital (SPDIF) audio outputs, though you would probably need a end card to capture that in any case.

Alternatively, you can get, or make an RCA volume control to bring down the volume a bit.

Something like this should work without affecting the sound quality, shouldn't it?

https://www.amazon.com/PAC-LC-1-Remo...s%2C161&sr=1-1

What is the clipped audio source?

The ES15 probably has AGC on the analog audio input to the recorder, but not on the pass-thru. This would reduce the chance of clipping if the audio input is hot. But it could also mess with the dynamic range of the audio.

BM Intensity documentation (page 19) indicates that it contains an audio proc amp (with about +/- 12 DB of gain) that can be adjusted using the BM Desktop Video Setup app. It also has a check box for "Use HiFi Audio Levels" if the RCA audio inputs are selected, in which case the level controls are not active. The proc amp woudl be in the analog audio path.

The source is as I said, both my listed VCRs. I also already tried setting the audio level control with the Desktop Video Setup program and trying the hi-fi setting and it still clips. I'm going to try an RCA volume control and see what that does to the signal.

Interesting.

I ran a quick test using bars and tone composite output from a Canon XA20 with the 1 kHz tone set to -12 dB. -12 dB corresponded to 0.30 volt rms measured by a high impedance multitester (or -10 dBV). It has the ~12 dB of headroom typical of consumer audio.

I captured the NTSC SD bars and tone with a BM Intensity Pro 4K card.
Used Desktop Video Setup input audio gain settings of 0 dB, +12 dB, -12dB, and HiFi.
I captured the XA20 output using BM's Media Express.

The audio levels in the captured files were -13 dB, -1dB, -25dB, and -1 dB relative to max allowable digital record levels as measured with Audition. (IMO this provides an adequate range of control for material that was recorded with close to proper audio levels, both consumer and professional.)

The HiFi setting corresponded to maximum gain in the internal audio proc amp and that may be excessive for material recorded or played back at a high audio level, as may be common with many VCR and camcorders using AGC for the audio input. (Material played back at level exceeding 0.30 volts might clip.)

The audio gain settings provided by the Intensity Pro 4K card and Desktop Video Setup worked as expected.

Sounds like your system is not functioning as expected, The simplest issue might be not saving the new setting (which in case of the internal card means allowing accepting to the computer).

Depending on its impedance the simple level control you linked to should work. However, using a simple external mixer provides metering that can help the workflow.

If something's not working right on my system, it's not working right across 2 VCRs, 3 different capture devices, and 2 operating systems. Hodgey has also seen this behavior. Your test is interesting, but is that signal large enough that it would be clipped before it gets to the gain control, as seems to be happening here?

I know basically diddly and squat about audio signals, so I have no idea what the culprit could be, but I ordered a simple level control from Amazon that should arrive today.

I will have to check with the intensity shuttle again, but from what I remember I had level issues with it. I didn't use the audio in much anyhow as I had issues with it creating random clicking sounds.

The VC500 and empia cards I've used have all had issues though, no matter what VCR I used.

As for levels, the Ag1980P specifies https://www.manualslib.com/manual/30...page=37#manual specifies -8 dBV output level. This consumer panasonic specifies -6dBV. The VR1100 we got specifies –8 dBs, 1k ohms (0dBs=0.775).

Slight chance it could be the VCR. (If the VCR is from me, then no, not it, because I check audio carefully.)

ATI 600 USB also have audio input levels problems, must use the registry hack to properly normalize at 50% (128 value). This hack is covered in various forum posts, for anybody needing it.

Audio quality is a main reason I use Turtle Beach Santa Cruz + ATI AIW desktops builds for capture. The ATI 600 clones strangely don't have the problem, nor do some of the specific model/revision Pinnacle cards I use.

- Apparently yet another reason not to like the VC500.
- And another reason that Blackmagic cards are craptastic for SD input.

Since talking about audio, and ES15, sometimes ES15 creates sync delays not found on ES10. Depends on signal quality, apparently buffers ramps up on bad input. So when using ES15, to be safe, I now route audio into it as well. Better than sync delay in VirtualDub later, quicker, less work.

LS, yes, it's with both the VCRs I got from you. When I use the ES15 for the video pass-thru I always route the audio thru it too just in case.

The audio level controller arrived, it works great. Interestingly with it in the audio signal path even with the level set to 0 (it has a +/- 8dB range) it helps. When I dial in a bit of reduction the audio levels are perfect with no clipping. Definitely worth $8.25.

Erich

Ran some more tests on audio recordign in the Instnsity pro 4K.

With SD analog audio selected:
- With the HiFi setting: it will not clip if the input remains at/under 0.22 Volts RMS. By 0.25 volts RMS it is clipping. This can be seen in the attached waveform graphic.

- with audio gain set to 0 dB: it will not clip if the input remains under about 0.98 volts RMS IN this case the captured digital audio is at -0.5 dB, essentially max record level.

- With audio gain set to -12 dB: it will clip when the input exceeds reaches about 1 volt RMS.

Bottom line is keep peak audio inputs below 1 volt RMS with the Intensity Pro.

Is that all stuff you can control with that Intensity 4K card? I don't know what most of that means, but at least I know I'm not the only one who was seeing clipping at times with multiple hardware. I captured a half hour tape last night with the level control and the waveform looks great in Audacity and Resolve.

The missing graphic.

Avoiding clipping requires understanding the capabilities of your gear and adjusting accordingly. If the purchased device works for you that is what counts.

There are a number of things you can control with the card's included software such as:
- Output type (HDMI, S-VIDEO, component, composite)
- Input (HDMI, S-VIDEO, component, composite)
- Analog video levels: Y, C, setup, Cb, Cr
- Audio levels (within ranges stated above)
- input/output conversions (SD to/from HD)
- Video format
- Capture file format
- And a number of operating parameters

If not set properly you can get garbage or perhaps no capture. If the user does not understand what these mean it is not the card for them.

I mainly use the Intensity Pro cards for ingesting HDMI signals and occasionally as a monitor connection point for output from my non-linear editor. The IP4K card has had significant issues with SD component capture levels.

That's basically the same adjustments available with the Shuttle Thunderbolt, which makes sense since they use the same setup and capture programs.

Basically in most cases, its just better to incorporate an analog mixer for proper sound levels. I always incorporate a mixer in my setup, as I use the Kona LHi to capture.

The RCA level control did the trick for me. However, I did file a bug report with Blackmagic about the Intensity Shuttle Thunderbolt and sent them a few test captures. They're taking it serious enough that they're going to have me send them my Shuttle to test to see if it's my unit, a software problem, or a design hardware problem.

Erich

I forgot to update this. Blackmagic returned my Shuttle Thunderbolt unable to recreate the problem, so I have no idea if it's their hardware or software, but either way, the level control takes care of it.

Erich

What's a reliable way to determine, during and then after capture, whether any audio clipping occurred?

The Panasonic AG-1980 has an audio level display. Will that help?

During capture I'm not sure how to do it, unless you're using a separate sound card that displays the waveform and can zoom out enough. Just seeing the audio levels won't do it. I've never done it that way, so I'm not sure if it would work. After capturing any program that displays the waveform you'll see the flat tops like in the screen shot in the first post of this thread. Using Virtualdub, just load the captured file and right click in the waveform after activating it to zoom out enough to see if the tops are flat.

http://www.digitalfaq.com/forum/atta...udio-levelsjpg

I don't think the graph shows every sample, just every nth sample, so I think the graph can only confirm that there *is* clipping, not that there *isn't* any.

In Virtualdub, select Volume Meter in the Audio menu. http://www.digitalfaq.com/forum/vide...html#post45240

You can view this in Preview mode before capture. Or, I think it can be viewed during capture in some cases. I have an ATI AIW capture card, and it already happens to be that I must be in Preview mode for capture to work. So I get it for free when capturing. Not sure if it works In Overlay mode. But, you may be able to capture in Preview mode without any negative effects. Experiment and find out. If not, you can preview the loudest parts of your tape before capture.

In Avisynth, command Histogram("audiolevels") shows audio levels as video plays. Refer to http://avisynth.nl/index.php/Histogram.

Sound Forge (not free) I know shows dB level along with the whole waveform. Audacity (free), I think would do that also. From virtualdub, load the video file, then save as wav and load that wav to one of those programs.

The meters on the AG-1980 (or any other device) can help but only after you understand the levels and limitations of each device through out the signal path, including the effects of any audio compression, limiting and automatic level control in the signal path. Clipping can be hard with flat tops and soft with somewhat rounded corners on the flat tops.

To make matters worse there is little consistency in published specifications among brands in the world of consumer electronics. And a pass-thru signal may be treated differently from a record/playback signal.

Clipping can happen at any stage in the audio path, and may not be evident on meters. The most reliable way to assess on the fly when shooting is with good headphones. In post opening the audio file in audio works station software like Audition will allow you to examine the waveform in detail and spot clipping.

Audacity has a feature to display clipping. https://manual.audacityteam.org/man/...#Show_Clipping

But this only shows samples that hit 0dB. In ELinder's screenshots, it looks like there is clipping despite having headroom.

If you install ffmpeg and point Audacity to it, you can drop any video file into Audacity and it will read the audio track(s). https://manual.audacityteam.org/man/...r_windows.html

Thank you very much for that information. In Audacity, I was zooming in all the way and didn't see any flat tops, I only saw them when I was zoomed out and getting confused. Now I know that the clipping was in the analog domain, not digital.

I've connected in a volume control to dial back the volume slightly, and it looks better on the waveform in Audacity, and it also sounds better. But I wonder if a pair of these attenuators would be better? Less noise, easier to reproduce the same audio capture levels every time instead of trying to find the same place on the dial?

Also, is it a good idea to normalize the audio after capturing as standard procedure?

In general, I do this, though only when restoring all of the audio at once.

And it really depends on the current overall loudness, as well as existing spikes. Normalizing isn't necessarily tied to loudness (nor volume, nor gain), merely adjusting levels to %. And that % is often arbitrary, being that it's not actually tied to loudness/volume/gain factors. Though it does affect volume/loudness (still not gain, as gain happened pre-capture).

Confused yet? :laugh:

I usually do 60% normalize -- but ONLY if peaks adjust upwards, not shrink. If shrinkage happens, you have a clipping/distortion issue to address.

BTW, the original issue in this thread sounds (pun not intended!) like a gain issue, either at recording time, in VCR, in capture card, or some other optional workflow device. That happens. Tracing/troubleshooting is required.

FWIW: I have encountered clipping that happened in the first stage of a recorder audio input. The input was set for a low mic level as with dynamic mics (typically -60 dBV output) but the connected mic was a self-powered condense mic (typically -35 dBV output). Thus the input signal was 25 dB higher than expected for the input configuration. Level controls that were located after the input stage were adjusted for normal VU meter indication with only occasional peaks above -10 dB indicated.

Later examination of the waveform (using Audition) showed clipped waveform peaks much like those in the graphic earlier in the thread. The clipped peaks were perhaps 6 dB below max digital record level. The problem was over driving the first input stage due to an incorrect setting of the recorder audio input.

It boils down to knowing what is happening in the audio chain during the initial acquisition both when making the original recording, and when capturing if from an analog source.

I just did a quick check on the audio output levels of the AG-1980.

With a 1 kHz sine wave recorded on a stereo track an output meter indication of 0 dB corresponds to about 0.3 volts open circuit. The meter's highest indication is +10 dB and that would correspond to about 1 volt RMS.

In light of the earlier post about the BM Intensity audio levels, audio where the AG-1980 playback meters go into the red (above 0dB indicated) will likely be clipped if using the Hi-Fi setting on the BM Intensity capture device.