DVX100 Audio Input

J.Beale   Nov. 17, 2002

The DVX100 mic input can be set to -50 dB or -60 dB input levels as a menu item, and the ALC (automatic level control) can also be turned on or off through CAMERA MENU-> 5. RECORDING SETUP-> MIC ALC. I am used to calling automatic gain control of anything just "AGC" but Panasonic reserves that term for video gain adjustment, and uses "ALC" for audio. I use the -50 dB mic level setting as my default, but you can choose -60 dB if you need the 10 dB extra gain for use with lower-output microphones. There are separate switches on the front of the camera which select mic or line level for each input. For these tests I used the mic input setting. The trimpots on the bottom edge of the camera adjust the left and right channel gain up to +11 dB (clockwise), and down to -36 dB (counter-clockwise) relative to center-straight up (0 dB).

trimpot scale

If you have the on-screen VU scale turned on, you'll see your current left and right audio levels on the screen. The visible scale marks start at -34 dBfs and the red part scale starts at -12 dBfs (from owner's manual, p. 48). The topmost (larger) red VU mark is less than 1 dB away from 0 dBfs, the full-scale point beyond which you get severe digital clipping. I aim to have the peaks just starting into the red territory a bit (-12 dB). Note the maximum record level, 0 dBfs (digital full-scale) corresponds to +4 dBu on the analog audio line-out (RCA jacks). If you have the ALC engaged it will limit your peaks to prevent hard clipping on anything but really excessive levels. With ALC off, there is nothing to protect you.

With ALC off, the mic input is linear right up to hard clipping at 0 dBfs (full-scale). With ALC on, the input is also linear up to -5 dBfs at which point the limiting circuit starts acting to reduce gain such that the input does not exceed -4.5 dBfs steady-state. Below -5 dB ALC does not affect the audio response, so I prefer to leave ALC on all the time. It will not hurt your audio if you keep your levels reasonable, and will save you from hard distortion if they aren't. I measured the ALC-mode to be capable of dropping gain at least 25 dB to avoid clipping. I did not measure time response, this was a steady-state test. From other live event experience, if your levels are really way too high the ALC can be momentarily overwhelmed and show some hard clipping.

The two plots below show the level of the analog line-out signal (RCA jack) based on a range of input levels. The camera settings were mic level = -50 dB, trim control fully clockwise (+11 dB position). In this case the total mic-in to line-out audio gain is +52 dB. If you set the trim at zero it would be be +41 dB, and with trim at minimum, +5 dB. If you instead chose the -60 dB mic level menu setting, and trim at +11 you could achieve a maximum mic gain of +62 dB.

For the plots I used a 1 kHz sine from a signal generator into the XLR input set to "mic" level. I measured input and output signal levels as shown on the graphs.  The digital clipping level (0 dBfs) corresponds to +3.87 dBu line output level, so any equipment set up for the standard +4 dBu "pro" level can handle the maximum audio output from the DVX100. Consumer audio gear set for -10 dBV (-7.8 dBu) line level might clip on the peaks.


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