Distorted audio

[bass_junkie83]

I am working on a simple video which when I play back has distortion on the audio.
It sound like crackling which seems to follow the sound patter, not sure of the correct term.

I was using a 5dmk2 with a rhode video mic. It's not clipping, but I don't know what has caused it.
How can I avoid it next time and is there any way I can reduce it in the clips I have?

I have uploaded a short sample video to show what the problem is.

 

[Steve Smith]

Was this quite loud in the venue? It sounds like the sound pressure level is bordering on the limit for the microphone so although you are not over-driving the input stage with too much signal, you are getting some mechanical distortion from the microphone itself.


Steve.

 

[bass_junkie83]

Yes it was an amplified performance so reasonably load.

Looking at the waveforms on audacity, the peaks are at around 0.15 so that shouldn't be clipping right? Is the mechanical distortion something different that wouldn't be visible by looking at the waveform?

 

[Steve Smith]

It's also possible that the mic preamp is being distorted before the level control. Either way, I think the distortion is being introduced before it gets to the recorder so even if you recorded it at a much lower level, it would still be present.

It's very difficult to see on a waveform as it's quite a subtle amount in a complex signal but it would show up as a waveform with a flattened top and bottom.


Steve.

 

[bass_junkie83]

I have been looking closer and there are flattened areas which coincide with where the distortion is most prominent.

Is it a case of completely wrong mic selection? A small diaphragm shotgun mic I imagine isn't the best choice, though it's all I had.

 

[Steve Smith]

It could be that your microphone was too sensitive for the sound level you placed it in. If it was designed for picking up normal conversation from a distance, using it close up with loud music would not be ideal.

Does it have a 'pad' switch on it? This would reduce its sensitivity, usually by -10 or -20dB.


Steve.

 

[bass_junkie83]

Yes it does have a couple of attenuation levels. I didn't really think of it at the time.
Is there a rough way to work out when you need to attenuate and by how much? Maybe I should to the unmanly thing and go read the manual.

Thanks for all your help. Next time I'll hopefully be more prepared, or even have a sound recorder I can just plug into the mixing deck!

 

[st599]

It could be that your microphone was too sensitive for the sound level you placed it in. If it was designed for picking up normal conversation from a distance, using it close up with loud music would not be ideal.

Does it have a 'pad' switch on it? This would reduce its sensitivity, usually by -10 or -20dB.


Steve.

If the diaphragm was distorted, would an electronic pad help?

 

[Steve Smith]

If the diaphragm was distorted, would an electronic pad help?

No.

Is there a rough way to work out when you need to attenuate and by how much?

There are two ways. The first is from experience, matching the attenuation to the level of sound. Do you have a VU meter to show the recording level? If so, choose an attenuation which puts your gain/recording level control at around the half way position

Another method is to use headphones to listen to what is being recorded (if you have that facility) although it might be difficult to separate what you are listening to in the headphones from the ambient sound if it is of a high level.


Steve.