Martin's audio & video problem-solving page
 I've been a regular contributor to user groups and technical forums for the last 20 years or so, and have written hundreds of posts offering information and advice on a whole range of computer, audio & video production topics. Trouble is, I never kept copies of any of them, which is why I've created this page. Martin Kay - May 2010 |
Q Desk Microphone - Recommendations?I'm currently recording a voiceover in Final Cut
Pro using a lapel microphone. Obviously this isn't an ideal situation. |
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A Martin writes...
You could always plug it into your camera?
Assuming it will take an external input. If not, budget in a very basic
mixer like a Behringer Xenyx 502 which can feed into the line-in of your
PC, via a long cable.
Here's a check-list for recording good speech.
1) A reasonably "dead" room, acoustically, so it doesn't sound like
you're in a room when the pictures might be of an exterior scene. Avoid
small, empty rooms with bare walls and lots of hard surfaces. The more
soft furniture, thick curtains, cushions and general clutter, the
better. If needs be, temporarily hang something like a duvet across the
room to damp down any resonances.
2) A reasonably quiet environment. It's pretty obvious you don't want
extraneous sounds.
3) Any half-decent microphone. In my experience, a mediocre microphone
in a good environment will produce a more usable recording than the
world's best microphone in a poor environment (ie one with bad
acoustics). Also, in that context, avoid placing the mic on a desk
stand, as the desk is a hard, reflective surface. Better to stand up and
use a floor stand (or even hand-hold the mic, with care) so that it's in
"free air", about a foot in front of you. I don't know what your budget
is, but there's a lot of relatively low-cost large-diaphram mics that
are intended for studio vocals recording, like the Behringer C1/C3 and
Samson C01 models, JoeMeek JM47, Rode NT1a, etc. Buy the best you can
afford, and it'll probably last you a lifetime. Most of my best mics are
over 25 years old.
4) Record at as high a level as possible without clipping! If it clips
(and distorts) you can't fix it in the edit - just record it again. If
it's way too low in recorded level then you compromise the signal to
noise ratio, and you can get quantisation distortion from not using
enough bits in the digital range.
Q What is the File size / Fps / Resolution dependency?Hello,
The problem I've encountered goes beyond my logic. |
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A Martin writes...
The answer is pretty much in the question, where
it's stated that "The only factor having much to say was bitrate".
Bit-rate is exactly that, the rate at which the data (in bits) is
generated (per second) by the codec. For example, if the bit-rate is
480Kb/s (four hundred and eighty thousand bits per second), and the
video was 20 seconds long, then the file size would be 20x480Kb (=
9600Kb). There are eight bits (small 'b') to a byte (big 'B'), and file
sizes are normally expressed in bytes (or kilo-bytes KB, or mega-bytes
MB), so the value of 9600 kilobits is divided by eight to get a value in
bytes. 9600 / 8 = 1,200KB or 1.2MB.
It is the bit-rate and duration that dictate the file size, nothing
else. So what effect do you get from changing the frame rate or pixel
resolution? Well, there is another equation which links those things to
bit-rate, along the lines of:- Pixel Resolution x Frame-rate x
Relative Quality = Bit-rate. If you want to increase the values of
any of the items on the left (Pixel Resolution, Frame-rate or Relative
Quality), you should increase the Bit-rate by the corresponding
percentage. If the bit-rate stays the same, then increasing one of the
other values (Pixel Resolution or Frame-rate) will cause a decrease in
quality from the codec, and visa-versa.
Of course some codecs are better than others, particularly newer ones
like H264, so simply using a different compression codec and keeping the
other values the same will change the relative quality of the encoded
video. Strictly speaking, the "Relative Quality" element is really
"Relative Quality/Codec Efficiency", such that a more efficient codec
will produce a particular subjective quality at a lower bit-rate than a
less-efficient codec. However, if you're already using the best codec
available for the job, it makes the equation simpler to leave it as it
stands.
Q How can I improve the quality, particularly volume, of a recorded piece of audio??Hi All, I have a problem that I need some
'sound advice' with! |
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A Martin writes...
Yes, I specialise in audio and, from what you
describe, the phrase that comes to mind includes the words "cat" &
"hell" and general negative connotations.
You don't describe in what way your attempt to lift the volume was
unsatisfactory, but I can guess. If the sound you're left with is from
the ME64 which was set up for the reading, then it was in the wrong
place and probably not pointing in the direction of the vows. That gives
you three problems.
1) The recording is low level. If that was your only problem, then you
could simply lift the volume to the right level. It might bring up some
electonic noise with it, but you might be able to reduce that with
software noise filtering and EQ.
2) If the mic is "off-axis", then it will have an uneven frequency
response to sounds coming from that direction, which can be difficult to
correct, given the complex EQ that would be required, and the fact that
any EQ you apply will affect the noise as well as the wanted sound.
3) If the mic is "in the wrong place" and further away from the source,
it will pick up a lot of diffused (reflected) sound, particularly from
the direction which is "on axis", to which the microphone is most
sensitive. These combined effects are known as "colouration" (if it was
light rather than sound, it would be the equivalent of colour fringing
and colour casts on the image). To remove it, you're up against the
physical laws of entropy, and in my experience it's a non-starter. EQ
won't help, and there's no "un-reverb" effect that I know of. It is
probably possible to enhance it for forensic purposes (ie simply being
able to hear what's being said), but it still won't be a nice sounding
recording - more like the comms sound on the Apollo space mission.
If it was possible to get a good recording from putting a microphone in
the wrong place and fixing it in software, no-one would bother to do it
properly and boom operators would no longer be required on dramas, for
example, but I don't see that happening yet.
Those, sadly, are the audio facts of life, in my experience.
Q Sound balance with two mics into my camera?As a 'newbie' I have a Sony HVR HD100E camera
which has a 3.5mm mic input that the on-board mic connects to (No
other)). I also have a Sony UPW V1 lavaliere mic receiver on the camera.
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A Martin writes...
Two thoughts on this.
1) Unless you're using the most rudimentary editing software, you can
adjust the individual levels of each channel in post. I think even
Premiere Elements can do this through the Channel Volume effect, which
is certainly present in every version of Premiere Pro.
2) If the track with the lower volume (the on-board mic) is really too
low to boost, then you need to attenuate the other input so that both
can be recorded at a decent level. This can be done with two resistors,
costing pence, some wire and and a soldering iron. You may have to pay
someone to do this, but it's not going to cost a fortune. As to what
values the resistors should be (ie how much attenuation should be
produced), you should look at the audio meters in your (audio or video)
editing software and see what the actual difference is (in dBs) between
the channels, or how much gain you have to add to the lower one to match
the higher one. From that an engineer can easily work out what values to
use. (This is a service that I can provide)
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