Specifications

CONSTRUCTION
S
ounds of various kinds have always
fascinated human beings. Many de-
vices have been invented for re-
cording and playing back the sounds—
from magnetic tapes to DVD (digital ver-
satile disc), from Adlib cards to high-per-
formance sound cards with ‘surround
sound’ capability. For personal comput-
ers (PCs), there is a wide variety of such
devices. A modern PC, generally, has a
‘Sound Blaster’ card installed in it. If your
PC does not have a sound card, here is
a low-cost audio playback circuit with
bass, treble, and volume controls to cre-
ate your own music player.
The playback device ‘M-player’ (i.e.
media player) described here uses mini-
mal hardware to achieve a moderately
good-quality audio playback device.
The software that accompanies the hard-
ware is meant for a PC running under
MS-DOS or a compatible operating sys-
tem. This device can play a simple 8-bit
PCM (pulse code modulation) wave file
with some special effects. The PC is con-
nected to the device through the PC par-
allel port.
Hardware
The circuit functions as an 8-bit mono
player, i.e. the sound files (with .WAV
extension) with sound quantised to eight
bits or 256 levels can be played. In
case of files with 16-bit quantisation, these
are re-quantised as discussed under ‘Soft-
ware’ subheading. Thus, only eight bits
are sent to the card through the printer
port.
Since there is no duplex communica-
tion necessary between the player card
and the PC, it is sufficient to use the eight
output data lines of the port 378H (pins 2
through 9 of 25-pin D-connector). This 8-
bit digital output is converted into an ana-
logue signal using DAC 0808 (IC1) from
National Semiconductor.
The output current from the DAC
varies with the input digital level
(represented by bits D0 through D7),
the reference voltage (V
ref
), and the value
of series resistor R1 connected to V
ref
pin 14 of DAC0808 IC. The output cur-
rent I
o
(in mA) is given by the relation-
ship:
where V
ref
is the reference voltage in volts
and R1 is the resistance in kilo-ohms.
The output current from the DAC is
converted into its corresponding voltage
using a simple current-to-voltage con-
verter wired around one part of the dual
wideband JFET op-amp LF353. The out-
put from IC2(a) is the required audio sig-
nal that has to be processed and ampli-
fied to feed the speaker. The part follow-
ing the I-V converter is the bass- and
treble-control circuit employing RC-type
variable low-pass and high-pass filters
connected to the input of audio amplifier
built around the second op-amp inside
LF353 [IC2(b)].
The frequency response of the filters
can be varied using potentiometers VR1
and VR2. The low frequencies or bass can
be cut or boosted with the help of poten-
tiometer VR1. Similarly, high frequencies
or treble can be cut or boosted with the
help of potentiometer VR2. At low fre-
quencies, capacitors C2, C3, and C4 act
as open circuits and the effective feed-
back is through 10k resistors (R4, R5,
and R6) and potentiometer VR1.
The audio amplifier IC2(b) acts as an
inverting amplifier and the amplification
(or attenuation) of the low-frequency bass
signals depends on the value of potenti-
ometer VR1. The frequency f1 at which C
= C2 = C3 becomes effective is given by
the equation:
Fig. 1: Circuit of M-player audio playback device
N.V. VENKATARAYALU AND M. SOMASUNDARAM
S.C. DWIVEDI
PC INTERFACED AUDIO
PLAYBACK DEVICE: M-PLAYER
75