16. How shall we avoid modulation / distortion on audio output when decoding
MP3?
Please take care of the digital volume of the MP3 decoder (MP3VOL register). When
MP3VOLL and MP3VOLR are set to maximum (1Fh), it is possible to have modulation
for a 0 dB sinus.
For testing audio output from sinus wave, use the default settings for MP3VOLL(0Fh)
and MP3VOLR (0Fh) (MP3 decoder digital volume) and adjust the LINE_OUT gain to
get 0dB on full chain.
In AT89RFD-08 firmware, it is possible to send the AUDIO_SET_GAIN (Line_out, 00h)
command (7Eh FFh 91h 02h 00h 00h 92h).
17. Could you please confirm the difference between 0dBFS (Full Scale) and
zero? (target characteristic is over 85dB.)
"Zero" has no signification. For SNR and THD parameters of the DAC, they are referred
to its full scale (dBFS). In the DAC specification, SNR and THD are given for a -1dBFS
level.
For the SNR, the Headset amp should be loaded with a 32 Ohms resistor (headphone
specification)
Coupling capacitor value: 100uF.
Test equipment: AUDIO PRECISION system Two 2322 for example or FFT Bandwidth
20KHz, A weighting.
Test Condition:
n DAC Gain (DAC_RLOG,DAC_LLOG) = 0 dB => AUDIO_SET_GAIN command for
DAC (00h).
n PA Gain (in PA_CTRL) = 0 dB => AUDIO_SET_GAIN command for PA (01h).
n Output load on Headset (HSR,HSL) = 10Kohms.
n Play a 1KHz -3 dB sinus MP3 on DAC.
18. How can I test the distortion characteristic on headset?
This distortion depends on load impedance: typical 32 ohms.
Use a 32 Ohms load (through a 100uF coupling capacitor)
Input signal: pure sine wave, level -1dBFS, frequency 1KHz
THD spec: -65dB typ, -60dB max
Test equipment: AUDIO PRECISION system Two 2322 for example or FFT Bandwidth
20KHz, A weighting.
19. Could you confirm the frequency characteristic on headset? (Headset capaci-
tors value are not yet defined)
The low frequency cut-off is determined by the capacitive coupling with the load. With a
100uF capacitor and a 32 Ohms load, the frequency cut-off is about 50Hz.