Assignment 1: Principles of Digital Audio and PCM Note: The following questions refer to the Digital Audio System as a whole and to the CD Player specifically. Not all the answers to these questions are found in the notes. Some further reading or research may be required!
1. Pulse coded modulation refers to: B (a) The quantize intervals of a digital audio system; (b) The process used to change an analog signal to digital; (c) The number of bits used in a system; or (d) NRZI modulation.
2. In a “Digital Audio System”,8-bit encoding would be suitable for: D (a) Speech only; (b) Hifi music only; (c) Speech and Hifi music; or (d) Low quality music and speech.
3. The number of ‘bits’ in a “Digital Audio System”refers to: C (a) The storage space available on the disc or medium; (b) The type of DAC used; (c) The number of quantize intervals used; or (d) Parts used in the CD player.
4. Quantisation error during encoding of an audio signal occurs when: D (a) The sample frequency is too high; (b) The quantize intervals are too many; (c) The audio input is greater the ½ fs; or (d) The sample taken is averaged out to a specific level.
5. The input low pass filter is referred to as an “anti-alias” filter because: A (a) It limits the audio input to ½ fs; (b) It introduces aliasing during recording; (c) It limits the sample frequency to ½ fs; or (d) It is actually a high pass filter.
6. With an audio frequency of 10 kHz and a sampling frequency of 44.1 kHz, the following alias frequencies are produced: B (a) 54.1 kHz 64.1 kHz (b) 54.1 kHz 34.1 kHz (c) 34.1 kHz 24.1 kHz (d) 10 kHz 44.1 kHz
7. “Foldover” of “alias” frequencies is a problem because: D (a) It requires too much space to store; (b) You cannot encode it to PCM signal; (c) The input low pass filters attenuation slope is too steep; or (d) It introduces a non-eliminable distortion in the audio.
8. One way of improving the quantity of a “Digital Audio System” would be to: D (a) Decrease the sampling frequency;\ (b) Reduce the number of ‘bits’ for every sample; (c) Reduce the audio bandwidth to prevent aliasing; or (d) Increase the sampling frequency.
9. The signal found at the output of the input sample and hole would be: A (a) A staircase signal; (b) Exactly the same as the input signal; (c) Normally distorted; or (d) Always a square wave.
10. An output sample and hold circuit is require to: B (a) Count the number of audio conversation; (b) Deglitch the output of the DAC; (c) Re sample the output to improve quality; or (d) Hold the Audio output constant during muting or pause.
11. Modulation of the digital “bit stream” is necessary to: B (a) Allow it to transmitted on to an antenna; (b) Reduce the bandwidth and increase data density; (c) Decrease data density on disc; or (d) Increase the bandwidth of the audio.
12. The following table highlight some main specifications of the Compact Disc digital audio system: C Sample rate No. of Audio Channels No. of Subcode Chs (a) 20 kHz 8 2 or 4 (b) 44.1 kHz 4 4 (c) 44.1 kHz 2 or 4 8 (d) 24.1 kHz 2 8
13. “Eight to Fourteen” modulation (EFM) is necessary to: B (a) Reduce the number of zeros between 1’s; (b) Prevent data loss; (c) Increase the number of 1’s between 0’s; or (d) Increase the data bit rate.
14. The “merging bits” added to the encoded data are: C (a) Used to reconstruct the audio data; (b) Used by the CD player for frame sync; (c) Discarded and become redundant in the CD player; or (d) Inserted by the error correction circuit.
15. The EFM rule states that: D (a) There can be no more than two 0’s between 1’s; (b) Frame sync is 24 bits long; (c) There are 588 bits per every frame; or (d) Pit sized are limited to 3T – 11T.
16. Merging bits are chosen to : A (a) Prevent data loss due to beam resolution; (b) Separate every 8-bit symbol; (c) Make the digital sum value (DSV) positive; or (d) Improve the operation of the NRZI Modulator.
17. The CD player requires frame synchronizing signal to: C (a) Read the audio symbol; (b) Create eight channel of subcode; (c) Spin the disc at the correct speed; or (d) Allow synchronizing to external devices such as Personal Computers etc.
18. The frame sync signal is not subjected to EFM because: D (a) Its frequency would be too high; (b) Its frequency would be too low; (c) It’s quality could be degraded; or (d) It has to be easily identifiable by the CD player circuit.
19. The frame sync frequency is: B (a) 588 Hz; (b) 7.35 Hz; (c) 75 Hz; or (d) 4.3218 MHz.
20. A compact disc recording of 74 minutes length contain: C (a) Approximately 1000 frames of data; (b) No error correction due to limited space on the disc; (c) Approximately 1000 million bits of information; or (d) No subcode in formation.
21. Each frame on the CD player contains: A (a) 6 complete audio samples ( L and R ); (b) 12 left samples of audio; (c) 2 pulses of sync; or (d) Two 8-bit symbols of P parity only.
22. In a CD player the frame sync signal is expected to be found immediately after the : D (a) Servo stages; (b) Optical block; (c) Digital to analog converter; or (d) Decoder stages.