Q: What are the specifications of the microphone port on the monochrome display? What about the built-in microphone on the monochrome monitors that come with the 68040-based computers?\
\
A: Both the external jack and the internal microphone run through a preamplifier and then are fed into a CODEC, which is an A/D converter designed for voice-grade phone applications. The CODEC compresses the digital signal using a method called mu-law compression and puts out an 8-bit coded signal. The dynamic range of a mu-law encoded signal is equivalent to that of a 12-bit linear (i.e., uncompressed) signal.\
\
The signal-to-noise ratio is limited by the CODEC to about 35 or 40 dB, although the built-in mike tends to pick up room noise. The frequency response is also limited by the CODEC to approximately 300–3500 Hz.\
\
Our internal microphone is an electret type which is quite sensitive and has a frequency response better than the CODEC.\
\
The external input is limited to about 1 to 1.5 mV rms before it soft clamps. Any other information may be best generated by experimentation.\