In using Alpha’s IC, there is often a need to measure the sound speed quickly. It’s time-consuming and inefficient if you measure the speed by directly playing the sound. Based on tests and analysis of different Alpha ICs, we come up with a quick method for measuring sound speed.
AM9 Series
The PWM frequency of AM9 series ICs equals to 4 times of the sound sampling rate.
For AM9AC \ AM9AE \ AM9AF \ AM9BB: PWM frequency = sampling rate×4. If the PWM frequency is 48.0KHz, then the sampling rate is 12.0KHz. All sounds use the same sampling rate, which is realized by adjusting the value of Rosc.
For AM9AD \ AM9BD: PWM frequency = sampling rate×4. If the PWM frequency is 33.2Hz, then the sampling rate is 8.3KHz. A fixed value oscillation resistor is used and there are 16 sampling rates to match different sounds. As PWM frequency changes with the sampling rate, the PWM frequency of different sound sections may vary, and that of the same sound may also vary if the subtables are different.
In measuring the PWM frequency for AM9AD and AM9BD, the value displayed on the oscilloscope is only the value for that specific sound section.
AM4E/4F Series
The PWM frequency for AM4E/4F ICs is in fixed proportion to its oscillation frequency, but not in fixed proportion to sound sampling rate. So the oscillation frequency can be calculated by measuring the PWM frequency. This is different from other ICs in AM9 series.
AM4E: oscillation frequency = PWM frequency ×128. If the oscillation frequency is 4MHz, the PWM frequency is 31.25KHz.
As the PWM frequency is not 4 times of the sampling rate, one sound sampling does not correspond to 4 PWM pulses. When a sampling happens, the PWM width must update. Thus, the pulse period in each 4 pulses is not the same.
In measuring the PWM frequency of AM4E/4F series ICs, the frequency of the first pulse displayed on the oscilloscope should be considered as the correct value.
AMSR Recording & Playing Series
Like AM9 series ICs, the PWM frequency for AMSR series ICs is 4 times of the sampling rate, while the sampling rate is determined by the oscillation resistor Rosc. During recording, the frequency of the BI sound equals to 1/2 of the sound sampling rate.
PWM frequency for SRAM series ICs = BI sound frequency×2 = sampling rate×4.
The sampling rate can be calculated by measuring the BI sound frequency during recording, without having to replay the sound and measure the PWM frequency.
