For AT+BTA1 and normal SCO, the data is transmitted once every 6 timeslots so the
transport delay is expected to be 6/1600 = 3.75ms. When doing loop-round testing with the
codec, i.e. with no transport delay, it was found that from input to output, the codec added
~1ms of delay at 1kHz and 1.5ms at lower frequencies.
4.4
PCM Timing
The codec samples at 8 kHz. The default mode of operation of the codec is 16 bit Receive
Gain Adjust Mode. In this mode, in every 8 kHz cycle, 16 bits of data is clocked into the
codec. The first 13 bits are PCM audio data, the last 3 bits are volume data. Of the last three
bits, 000 equates to maximum volume (ATS589=7), 111 equates to minimum volume
(Ats589=0).
At maximum volume, the output signal matches the amplitude of the input signal at the
other end of the
Bluetooth
link. It is more appropriate to think of this feature as being an
attenuation control.
The clock rate used for sampling is 250kHz (4µs). 16 clock cycles takes 64µs. 8kHz equates
to 125µs.
The same timing is used for all packet types in both SCO and eSCO modes.
5.
Frequency Response
5.1
Codec Frequency Response
The codec frequency response can be measured by connecting PCM_IN from the codec to
PCM_OUT to the codec (PCM_OUT from J1, the 10 way connector has to be disconnected). A
1k
Ω
pull down resistor is needed on PCM_OUT to ensure maximum volume setting.
The following graph shows the measured frequency response. For this test, R32, the side-
tone resistor was removed to prevent audio feedback.
A 1V peak to peak sine wave was injected into the microphone circuit and its amplitude
measured at TP5, A0, the input to the codec. The output from the codec was measured on
TP6, PA0+.
The chart below shows the codec frequency response with the High Pass Filter Enable (HB –
Pin 16) pin set high and set low.
Frequency Response of Codec (R32 Removed)
0
0.2
0.4
0.6
0.8
1
1.2
0
500
1000
1500
2000
2500
3000
3500
4000
Fre que ncy (Hz)
PA
O
+
(
V)
HB=1
HB=0
APN_06002_1v03 Winbond Audio Board Application Note
7