The object of recording diphones is to get as uniform a set of
pronunciations as possible. Your speaker should be relaxed, not be
suffering for a cold, or cough, or a hangover. If something goes wrong
with the recording and some of the examples need to be re-recorded it is
important that the speaker has as similar a voice as with the original
recording, waiting for another cold to come along is not reasonable,
(though some may argue that the same hangover can easily be induced).
Also to try to keep the voice potentially repeatable it is wise to
record at the same time of day, morning is a good idea. The points
on speaker selection and recording in the previous section should also
be borne in mind.
The recording environment should be reconstructable, so that the
conditions can be set up again if needed. Everything should be as
well-defined as possible, as far as gain settings, microphone distances,
and so on. Anechoic chambers are best, but general recording studios
will do. We've even done recording in an open room, with care this
works (make sure there's little background noise from computers, air
conditioning, outside traffic etc). Of course open rooms aren't ideal
but they are better than open noisey rooms.
The distance between the speaker and the microphone is crucial. A head
mounted mike helps keep this constant; the Shure SM-2 headset, for
instance, works well with the mic positioned at 8mm from the lips or so.
This can be checked with a ruler. Considering the cost and availability
of headmounted microphones and rulers, you should really consider using
them. While even fixed microphones like the Shure SM-57 can be used
well by professional voice talent, we strongly recommend a good headset
mic.
Ultimately, you need to split the recordings into individual files, one
for each prompt. Ideally this can be done while recording on a
file-by-file basis, but as that may not be practical and some other
technique can be used, such as recording onto DAT and transferring the
data to disk (and downsampling) later. Files might contain 50-100
nonsense words each. In this case we hand label the words, taking into
account any duplicates caused be errors in the recording. The program
ch_wave in the Edinburgh Speech Tools (EST) offers a function to
split a large file into individual files based on a label file. We can
use this to get our individual files. You may also add an identifiable
noise during recording and automatically detect that as a split point,
as is often done at the Oregon Graduate Instititute.. They typically
use two different noises that can easily be distinguished and use one
for "OK" and "BAD" this can make the splitting of the files
into the individual nonsense words easier. Note you will also need
to split the electroglottograph (EGG) signal exactly the same way, if
you are using one.
No matter how you split these, you should be aware that there will still
often be mistakes, and checking by listening will help.
We now almost always record directly to disk on a computer using a
sound card; see Section 4.6 for recording
setup details. There can be a reduction in the quality of the
recording due to poor quality audio hardware in computers (and often
too much noise), though at least the hardware issue is getting to be
less of a problem these days. There are lots of advantages to
recording directly to disk, as the stage of digitising, transfering
and spliting the offline records is laborious and prone to error.
