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4.4
Audio
The BitsyXb includes an AC'97 codec for stereo audio input and output. Electrical specifications
for the audio system are listed in section 6.3.8.
4.4.1
Microphone Pre-amps
The BitsyXb supports the connection of a stereo electret microphone to the MIC_R and MIC_L
inputs on J10. The audio signals run through pre-amplifiers that low-pass filter and boost the
signal before being passed on to the audio codec.
When connecting external electret microphones to the BitsyXb, use the MIC_GND analog ground
plane for improved signal-to-noise ratio. The BitsyXb includes pull-ups to power electret
microphones.
4.4.2
Audio Outputs: Speakers and Headphones
The BitsyXb audio amplifier supports both differential and single-ended output devices.
Differential (or "bridge") drive delivers greater output power and is suitable for speakers, which
can be wired independently from each other. Single-ended mode is used for devices like
headphones, which have a common ground between output channels.
The HP_IN input (J3.32) determines the output mode of the amplifier: When HP_IN is high, the
audio output drive is single-ended, when HP_IN is low, the output drive is differential. An on-
board pull-up normally keeps HP_IN high.
Connecting Speakers
When using the BitsyXb to drive speakers, short the HP_IN signal to ground. This places the
output amplifier in differential mode. Connect speakers to the SPKR_L and SPKR_R outputs on
J10.
Connecting Headphones
Standard headphones use a plug wired as shown at
right. Three rings on the plug provide right and left
channels and a common return. Mono headphones
do not include the center ring.
The mating headphone jacks include spring contacts to make an electrical connection with the
headphone and to mechanically hold the plug in place. Some jacks include a mechanical switch
suitable for use with the HP_IN signal that is activated when a plug is inserted into the jack.
Mono Headphones
You can connect mono headphones directly to the
BitsyXb as shown at right. Keep in mind that the
resulting impedance of the parallel-connected
headphone speakers is half that of a single
headphone speaker. See the audio driver
specifications in section 6.3.8 for details about the
minimum impedance an audio output channel can drive.
