New to Audio - Help with Understanding Recorder

T

taliamr

New member
We are new to the audio side of video, and I have a few questions that I am hoping to get some answers or suggestions to! We are filming video with the Canon 5D Mark II, and when it comes to audio, we are mainly recording vocal interviews with people. We have been using a Rode Shotgun Mic on a Boom and recording into the camera through a JuicedLink box. This had been working rather well, we were recording external secondary audio onto a Zoom H4N for backup too.

However, we had 2 occasions where we were getting radio frequencies (Music from a radio station) coming through the JuicedLink. We could hear it on the headphones, and it was being recorded into the same file as the vocal audio from our subject. We did some research, and asked around and didn't find anything about this problem...it seemed crazy! So, we just started looking into other options other than the JuicedLink. Anyone ran into this problem with the JuicedLink?

We recently purchased a Sound Devices 702 recorder, and I am trying to wrap my head around how to use this on set. We have figured out the basics of the recorder, but have some detail questions, such as 'Low-Cut Frequences/ Filter' -- when to use them, and what filter to use, etc. On location, we have had trouble with air-conditioners and other outside noise, and wonder what combination of low-cut frequency filter would be best for different situations?

I would love to be able to attend some sort of workshop for audio, but have been unable to find much of anything available. Basically, I need to learn the basics of audio for recording to DSLR so that we are able to problem solve on set. I am thinking of purchasing the book 'Location Audio Simplified' but if anyone has any other suggestions on books / tutorials / workshops etc. I would really appreciate it!

Thanks,
Talia
 
Welcome to DVXUser, Talia!

Moving from in-camera DSLR audio to the H4n is a pretty big step in terms of quality to begin with, as DSLR audio is generally pretty poor with or without AGC. But moving up to the SD702 is a jump into the high-end world of sound recording, so kudos!

I can't tell you personally if the JuicedLink issue has come up before, but I can take a guess that there's a grounding/shielding problem somewhere in your audio chain. If you've isolated it to the JuicedLink, meaning that the NTG-2 straight into the H4n does not get the same problem in the same location, then you can be sure that something is wonky inside that box. However, since staying away from in-camera DSLR audio is highly recommended, and you have the SD702 now, this is pretty moot.

As for the SD702 and filters, just remember that your goal in production is to record as cleanly as possible with as little manipulation to the signal as possible. Save EQ and compression for post-production. Low-cut filters can be life-savers, though. They are used (usually around 80Hz or 100Hz) to get rid of low-end rumble from distant air conditioners, refrigerators, highways/Interstates, etc. But before you go slapping on some filters, do everything you can to secure the location for sound. Refrigerator running? Turn it off, but put your car keys inside so that you remember to turn it back on before you leave. AC running? Turn it off during takes, and run it during breaks. Distant roadways can be tempered using the off-axis rejection of the shotgun, though a bit of low-cut may be necessary still.

Also, if you're recording just the one mic, set your second record channel about -6dB lower than the first. This is your safety channel in case of sudden peaks. You only need one channel (track) of that audio in post, so use the primary track until there's clipping, then fall back on the safety.

Last, I'd highly recommend THIS DVD to get you through the basics.

Good luck!
 
Thanks for the reply and recommending the DVD...I'll definitely check it out!

Regarding the low-cut frequency filters on the SD702, we have been in many situations so far where we are shooting interviews in large office buildings where we have no control over the air conditioner and other annoying noises, so that is why we are so interested in learning about these filters. But, our main question right now, is which filter to use? The SD702 has several choices (40Hz / 80Hz / 160Hz / 240Hz all at either 12dB, 18dB or 24dB/ OCT. What would be the reason to use 80Hz at 12dB/Oct versus 160Hz at 24dB/Oct?

We did some tests, and obviously the higher Hz we get (160 or 240Hz) the better it cuts out the background noise, and when we changed the dB/Oct it didn't seem to make much of a difference. But we know that there has to be a reason for the choices.

Talia
 
For starters, using an NTG-2 in your run-of-the-mill office building is going to be problematic. As a general rule of thumb, a shotgun mic is the wrong tool for interiors. Long story short: the interference tube, which is what gives the shotgun mic it's off-axis rejection, works against it in reflective environments. It cannot reject bass frequencies from any direction, and between that and the reflections from the room, the sound is going to be roomy, muddy, and pretty low-end heavy. The better choice in situations like that is a hypercardioid condenser (AKG CK93, Audio Technica 4053b, Oktava MK012). But for seated, talking-head interviews, you may be better off with a hard-wired lavaliere mic (Countryman EMW or B3 or B6, RODE Lav, Sanken COS11). If you want more info on why shotguns aren't great indoors, do a search as it has been discussed in-depth around here. The DVD also covers this issue.

Also, the mic needs to be close - very close - to the source. Get it as close to the interviewee as possible without it being in frame, and aim it at the sternum. The closer the mic is to the source, the higher the ratio of dialog:background is recorded, meaning that background noise will be less noticeable. An effective working distance for any boom mic is generally 2' at most, but the rule still stands at "the closer, the better." 2' is pushing it. 12" is much better. And there's an inverse square law at play here which says that half the distance to the source means 4 times the amplitude. Twice the distance, 1/4 the amplitude.

As for low-cut options, you don't really want to go higher than 160Hz as it will start affecting the voice in adverse ways. In fact, if you can get away with 80Hz you'll be better off. As for the dB/octave, that determines how steep the roll-off is. An octave in sound frequencies is either a doubling or a half of the base frequency. So 12dB/octave at 80Hz means that the attenuation will be -12dB at 40Hz and -24dB at 20Hz. The steeper the roll-off, the more drastically low-frequency information is cut. How steep the roll-off is really determined by how much low-end interference you need to remove.
 
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Another question I Have regarding the Sound Devices 702 is the levels. We understand that when recording the levels should be hitting at -20dB, is this correct?
 
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