The information depth of A/D converters or D/A converters is the product of the sampling rate and the quantization. If one increases the sampling rate, the digitization can be reduced without affecting the information depth. This basic consideration is one aspect for oversampling. Another aspect is related to the problems of filtering and signal smoothing.
According to the sampling theorem, when a frequency is digitized, it must be sampled at at least twice the sampling rate. For example, for compact discs, the sampling rate is 44.1 kHz, which results in a maximum audio frequency of 22.05 kHz. To prevent the harmonic and mixing products from affecting the signal at such a sampling frequency, the audio region must be separated from the mixing products from the clock frequency using filters that have a high slope. This causes problems because filters with high slopes cause phase shifts in the upper frequency range.
For this reason, one multiplies the sampling frequency and can thus use normal filters with lower edge steepness. In addition, oversampling smoothes the stair-stepped signal. For example, with a tenfold higher sampling rate, ten more intermediate values could be interpolated per staircase step. The analog signal would thus become more stepless and then be further smoothed in a low-pass filter. The playback of the analog signals is limited to the playback range by the low-pass filter.
In D/A converters, oversampling is used to achieve interpolation between two existing digital words. For this purpose, additional digital words are inserted between the existing ones. For example, if the input of an A/D converter is clocked at 44.1 kHz, then 4 times oversampling inserts three new samples and the clock frequency increases to 176.4 kHz. The oversampling ratio( OSR) is then 4. The faster sample stream is passed through a digital filter where the inserted samples are interpolated with the original data words. The analog signal is reconstructed with a D/A converter operating at 4x sampling rate and filtered out via a conversion simple filter.