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QUAL-SOFTWARE  August 2004

QUAL-SOFTWARE August 2004

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Subject:

Re: digital vs mini-disc recorders

From:

Sally Stone <[log in to unmask]>

Reply-To:

qual-software <[log in to unmask]>

Date:

Wed, 25 Aug 2004 11:24:51 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (104 lines)

Alan,
Does everything you shared here apply equally to Mac and non-Mac
users. I am a Mac user.
Sally

>DIGITAL AUDIO
>
>Going digital gets rid of the noise inherent in using tape. However, if you
>are going digital, there are several issues you should consider that will
>impact audio quality: sampling frequency, resolution, compression.
>
>If you want to capture a frequency accurately then you need to have a
>sampling frequency that is at least double. The limit of human hearing is
>around 20KHz hence the sampling rate on CD quality audio is 44.1kHz. (If you
>want to know more about this do a Google search for "Nyquist frequency").
>But, assuming you are not interviewing opera stars who will give impromptu
>performances during the interview, you don't care about recording the entire
>range of frequencies that the human ear can hear. Normal speech frequencies
>are in the mid-range i.e 250 Hz to 8kHz. If you are just interested in
>speech  you probably don't want the low frequencies or high frequencies
>outside this range. If you are recording speech for transcription the low or
>high frequency stuff is just noise and it will impact the intelligibility of
>the speech you are recording. In previous posts when people mentioned noise
>reduction all they mean is that the microphone isn't sensitive to
>frequencies at the upper and lower ends or the recorder doesn't encode those
>frequencies. You can also get this kind of effect after the fact by using an
>audio editing program like Audacity that can be used to filter out entire
>frequency bands. Given the Nyquist issue, this means one probably should use
>a sampling frequency of at least 16kHz for the best quality. I use 22.05kHz.
>Now you can get away with less. Telephone systems generally limit
>frequencies to the most critical frequencies for intelligibility, usually
>400Hz and 3.4kHz. Similarly with a lot of digital voice recorders--you'll
>find many have fairly low sampling rates and will only be good at recording
>frequencies up to 3kHz or 5kHz. Personally, if you are going to listen to
>this material over and over or transcribe it, I'd want more than the minimum
>quality for intelligibility.
>
>Practically all digital recorders use at least 16 bits to encode audio (i.e.
>the resolution, the value that can be given to each sample). I would avoid 8
>bit. The devices that plug into the iPod are 8 bit. It may get the job done
>but don't expect wonders.
>
>Compression is another issue. Uncompressed audio (PCM) takes up a lot of
>space. So people use codecs like MP3, ATRAC (what Minidisc uses) etc. to
>compress the audio. If you are a linguist you might desire to use
>uncompressed audio but MP3, ATRAC, WMA, OGG and other popular codecs will
>work very well at encoding speech even at fairly high compression ratios.
>The sample rate and resolution are generally more important considerations.
>
>MICROPHONES
>
>You probably want an omnidirectional microphone as those are the easiest to
>use for this type of work. Some mics are designed for recording speech, i.e.
>have a frequency sensitivity geared to the mid-range. You'll also find
>boundary or pressure zone microphones that are good for recording meetings
>and other events where the mic can be positioned on a large flat surface.
>These boost direct sound waves by 6db over reflected sound waves and thus
>aid intelligibility in situations where there might be a lot of reflected
>sound from walls and other surfaces. In some situations using multiple mics
>and stereo recording might prove very helpful.
>
>TECHNIQUE
>
>The number one key to intelligibility regardless of what you are using is to
>make sure that the speech you are recording is at least 30db louder than any
>background sounds or noise (40-50db is better) while avoiding 'clipping'.
>Microphones by themselves won't perform miracles in this regard. Lots of
>microphones will work reasonably well. What's important is that you get the
>microphone reasonably close to the speaker or speakers. Every time you
>double the distance of the microphone from the source you drop the audio
>relative to background noise by 6db. Move the microphone far enough away
>(and it doesn't have to be very far) and the speech you are recording will
>sink into the sludge of background noise. And you'd be surprised how much
>noise there is in a 'quiet' room. A previous poster suggested lapel mics.
>Good idea if they aren't perceived as intrusive and you don't run into a
>clothes noise issue. The other strategy is to limit ambient noise as much as
>possible.
>
>RECORDERS
>
>Voice Recorders ($200-$300). Some new models have much better sample rates
>and record in formats that can be readily used by many types of software.
>Check out the newest Olympus models (DM-10, DM-20, DS-2200). They have a
>best quality mode that samples at 44.1kHz, has a frequency sensitivity of
>300 to 8,000 Hz, and encode in WMA.
>
>Minidisc ($200-$400). New Hi-MD models will record in ATRAC at various
>compression ratios as well as PCM. You'll get 44.1 kHZ, 16 bit, stereo
>quality audio. Very good quality audio. Downside is that Sony still hasn't
>allowed rapid transfer to PC in a format that can be unlocked. Apparently
>they will release some type of converter that will allow this in the fall.
>As a previous poster pointed out, if you are going to a remote location MD
>may be the way to go as you can take lots of discs (which are fairly cheap)
>and don't have to bother immediately offloading your recordings as you would
>with some other types of digital recorders.
>
>Marantz PMD-670 ($600-$700). Uses CF cards. Sophisticated professional
>recorder. Records in PCM, MP3 and MP2 at sample rates up to 48kHz. Supports
>professional mics, selectable frequency band filtering, etc.  Well designed,
>easy to use. Very high quality audio.


--

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