To obtain high quality prompts, make sure you start with a spotless master recording. Here are a few common-sense guidelines that will make it easier to obtain good quality "masters". Only spotless recordings convert well and make good prompts. Many ingredients influence the quality of a recording: the microphone, the room's acoustical characteristics, and the position of the microphone with respect to the room and the speaker all play a very important role.
Use a hyper-directional microphone to avoid recording surrounding noises (PC fan noise for instance) and room echo. Do this even if you have a silent studio area. Better microphones will also have a lower inherent (electrical) noise factor. Cheap microphones are usually not directional and pick up all sorts of noises and room echoes, giving the recording a very amateurish sound.
Use a low-impedance hyper-directional microphone and keep mic cables short, shielded and away from power cables and outlets. Ground your PC system at one point only. Hum (at 50 or 60 Hz) in a lab or office environment is an interference that plagues many aspiring "sound engineers". If possible, do not record frequency content below 100Hz; it does not get transmitted over the telephone network bandwidth anyway.
Avoid recording in concert halls, empty basements or stuffy closets. The room's reverberation time and echoes have a very palpable influence on sound "quality". For simplicity, imagine reverberation being the time it takes a hard clap with the hands to die out (decay) until you can't hear it anymore, while echo is the fact that you can actually hear a second or maybe even a third clap. The reverberation time depends on the structure of the walls, floor and ceiling. The more they absorb sound, the shorter the reverberation time will be. The ideal situation is when the reverberation time is short and independent of the sound's frequency. This is very difficult to achieve in a normal office environment (but exists in professional studios), so you will have to compromise. Avoid large rooms without furniture and with glass windows (glass absorbs low frequencies and reflects high frequencies). If you have glass windows, use curtains to absorb reflections. Avoid unfurnished rooms with wall-to-wall carpeting and drapes over the walls (these only absorb the higher frequencies). Echo is something that is very disturbing and should be avoided by all means. The larger the room, the larger the echo risk. In the selection process for a studio room it is a very good idea to clap loudly with your hands and listen for sound decay and possible echoes. Move around in the room and clap your hands to determine how this changes the room response and where the best location is. Your colleagues may think you have gone nuts, but ignore them. This may seem strange but square or rectangular rooms with flat ceilings are the worst because standing acoustic waves are created in all three dimensions; asymmetrical rooms with sloping walls and furniture are better because standing waves are less likely to be created. In conclusion: you are unlikely to find a perfect studio room in your office, so establish your recording microphone in a well-furnished, preferably asymmetrical, mid-sized room where a clap with the hands decays in less than half a second, use a directional microphone and forget about the rest.
The spatial impression and richness of a recording depend on the location of both the microphone and the speaker in the recording room. The distance between the speaker and the microphone influences the spatial impression because you change the ratio between direct sound and reverberated sound. The closer the speaker is to the microphone the closer he will sound but his voice will also seem warmer because low frequency content will increase. The further the speaker is from the microphone the more spacy and distant he will sound and the more you will hear room echoes. The above is also a function of the directionality of the microphone, therefore trial and error recordings are needed to determine the optimal position of the speaker with respect to the microphone. For speech, the speaker at arm's length distance from the microphone is usually about right for a neutral impression. In any event, keep the microphone far enough from the speaker's lips to minimize "plops" and breathing sounds. If your microphone rests on a table, try and minimize the interference between direct sound and sound reflected by the table surface by placing the microphone as close to the table surface as possible. Another way of avoiding this is to use a highly directional microphone (again).
Another, very important factor is the quality of the voice itself. Use experienced speakers with a clear pronunciation and telephone-friendly voice. A voice that sounds great on radio or TV may not sound all that great over the telephone system. Perform preliminary tests. Some speakers have a propensity to produce DTMF sounds; avoid their services. Other speakers pronounce the letter S as if they were ssschnakes trying to hypnotize a prey--not good either. Test the speakers' voice quality before and after conversion to the desired telephony format and play a few test prompts over the telephone network via a voice telephony card.
Use a PC with no fan (e.g. a portable PC) or one with a very quiet fan. Keep the directional microphone at a distance and directed away from the PC.
Use a quality sound card in a PC with a well-filtered power supply. Use a well-shielded, low emission monitor.
Avoid recording with an 8-bit sound card or with a 16-bit sound card set for 8-bit resolution. We have only included 8-bit capability in Vox Studio for those rare occasions where you absolutely need to use an existing 8-bit resolution file and do not care about quality. Otherwise, you should stick to 16-bit recordings.
If you hear hissing noises in your recordings that cannot be related to surrounding noise reaching the microphone, you are having electromagnetic interference problems. Swap your sound card for another, better one. Move the sound card to another free slot in your PC. Try another monitor. If that does not help, try swapping your PC platform for one that generates less interference. Make sure no hissing noises are produced while you move the mouse. Once you have found a clean combination of sound card, mouse, monitor and PC, stick with it.
Do not record in a room where others work. Close your window to keep outside noise away. Avoid recording under the flight path of Concorde. Record in the evenings, at night or during weekends if you cannot avoid hearing slight workday noises. Even slight noises get recorded and cause problems later.
You would be amazed at how many little noises actually disturb a place you think is quiet. The ticking of the clock on your office wall, the humming of your fluorescent lights, hot water flowing through your heating system, the air conditioning, a noisy hard disk, the cooling system of your PC. All of these add up. Stay in the recording room for two minutes, close your eyes and listen with acute attention. Do you hear anything at all ? If you do, select another studio, because your microphone is likely to hear it too.
Make test recordings with silence only. Do this in the recording room, with the PC, sound card and microphone you want to use. Place the mike on the table and start a 30-second silence recording at 16 bits and 44 KHz. Play this recording back through headphones, not through speakers. Listen with great attention. Do you really obtain absolute silence only? Look at the recorded signal in dB view. Is everything that is visible below -60dB or less? If not, solve this problem before you go one single step further. Clean recordings convert very nicely to other formats, recordings with background noises or hissing don't.
Record at reasonable levels. You must absolutely avoid compensating for low-level recordings by pumping the gain up at a later stage in the process! Avoid saturation while recording, except perhaps for very brief peaks.
Use the keyboard gently, so as not to record clicks. If you move a mouse on the desk that carries the microphone use a good mouse pad. Do not locate the keyboard on the desk that carries the microphone or do use a thick sound-absorbing mat. Avoid kicking the recording table. An office chair on wheels can generate noise when you move. Chairs with springs and floppy armrests can be pretty bad at making noises too. Test this.
One procedure to obtain good quality telephony prompts (there are many variations) would be to use a sound card and Vox Studio to record the prompts as ".wav" files at, say, 16 bits and 22 KHz, then use Vox Studio to convert those files to one of the telephony types, for instance A-law at 8 KHz. Avoid recording directly to 8-bit resolution files.
If you cannot follow the above guidelines, have a professional studio record the ".wav" files for you, or have them record a tape, DAT, mini-disc or Audio CD and use the Vox Studio Tape Loader to digitize their recording and convert the files to telephony formats and sample frequencies.
Always test the quality of your master files before proceeding to convert them.
Be patient and start over until you are absolutely proud of your original master recordings. Never go to the next step until the previous step yields perfect results. Never try to correct imperfect master recordings later using filtering, normalization or other conversion manipulations.