When users use SRAM Voice Recording IC (same model) to record, they find that the recording durations are different for the same IC (Differences are up to 10%). After taking a test for it, we found that the higher the volume, the shorter duration will be. For example, if the volume exceeds 90dB, the recording duration will be shortened by 20%.
These differences of durations are caused by two methods of compressing sound used in IC. One method is for compressing sounds with high volume; the other is for low volume. If the volume become higher, it will require more SRAM to store. So the duration become shorter, and also the sound quality is better. On the contrary, the lower volume requires fewer SRAM. Therefore, the durations are different. In this way, not only sound quality can be improved, but also much SRAM can be saved when recording low volume.
Hence, only if the IC records in mute condition and records the same sound, the recording durations are the same and the longest. So we should measure the accurate recording duration (defined in IC specification) in mute condition, otherwise the result is incorrect. There are also another 2 ways to measure accurate duration, they are: measure the BI frequency when recording or measure the PWM frequency when playing.
