The digital transmission of speech may be represented by the block diagram in the figure below.
State the purpose of the parallel-to-serial converter.
Part of the signal from the microphone is shown in the graph below.
The ADC (analogue-to-digital converter) samples the analogue signal at a frequency of 
. Each sample from the ADC is a four-bit digital number where the smallest bit represents 
. The first sample is taken at time zero. Use the graph above to determine the four-bit digital number produced by the ADC at times
