Designing and building a USB sound card
is no longer a head ache because we have got the PCM 2702 integrated
circuit from Texas Instruments. The PCM2702 is an integrated 16 bit
digital to analog converter that has two digital to analog output
channels. The integrated interface controller of PCM2702 is compliant to
the USB 1.0 standards. The IC can handle sampling rates of 48 KHz, 44.1
KHz and 32 KHz. The IC also has a number of useful features like
on-chip clock generator, digital attenuator, play back flag, suspend
flag, zero flag, mute function etc. The most interesting thing is that
this circuit is plug & play and doesn’t need any driver software for
Windows XP and Windows Vista operating systems.
The circuit gets control data and audio
data from the USB through the D+ and D- pins of the PCM2702 all the data
transferring is carried out at full speed. The decoded audio signals
will be available at the VOUTL and VOUTR pins of the IC. The 12MHz
crystal is connected between the XT0 and XT1 pins of the IC. The VBUS
(USB bus power) pin and DGND (digital ground) pins of the IC are
connected to the +5V and ground pins of the USB respectively. The
circuit requires +5V DC and +3.3V DC for operation and both of these
voltages can be derived from the USB port using LDO (low drop out)
voltage regulators (not shown in circuit).
Notes.
- +5V DC supply can be derived from the USB port using a +5V LDO regulator.
- +3.3V DC supply can be derived from the USB port using a +3.3V LDO regulator.
- Audio signals (output) available at VOUTL and VOUTR requires further amplification for driving low impedance head phones or loud speakers.
- The PCM2702 is available only in SSOP28 package and requires special care while assembling.
- Before attempting this circuit please go through the datasheet of PCM2702 and get a clear idea about the device.
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