<p>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!</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>19. The frame sync frequency is: B (a) 588 Hz; (b) 7.35 Hz; (c) 75 Hz; or (d) 4.3218 MHz.</p><p>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.</p><p>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.</p><p>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.</p>
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