Question

Question 1 a) A digital sound recorder captures sound from the real-world, processes the audio and stores it in binary form. Using the principles of digital signal processing, practically explain how this phenomenon occurs. AP 5

Answer 1

Digital Sound Recording:

• It is a method of preserving sound in which audio signals are transformed into a series of pulses that correspond to patterns of binary digits (i.e., 0’s and 1’s) and are recorded as such on the surface of a magnetic tape or optical disc.
• A digital system samples a sound’s wave form, or value, several thousand times a second and assigns numerical values in the form of binary digits to its amplitude at any given instant.
• A typical digital recording system is equipped with an analog-to-digital converter that transforms two channels of continuous audio signals into digital information, which is then recorded by a high-speed tape or disc machine.
• The system uses a digital-to-analog converter that reads the encoded information from the recording medium and changes it back into audio signals that can be used by the amplifier of a conventional stereo sound system.
• Digital recording provides higher-fidelity sound reproduction than do ordinary recording methods, largely because audio signals converted into simple pulse patterns are virtually immune to the residual noise and distortion that are characteristic of analog communication channels and sound recording media.
• In addition, many digital recording systems are designed to detect and eliminate interfering signals.
• In the 1980s digital compact disc recordings became available that were played by using a laser beam to optically scan digital information encoded on the disc’s surface.
• In the late 1980s digital audio tape (DAT) recorders using magnetic tape cassettes became available for audio reproduction and recording.
• The DAT recorder converts audio signals into digital data on a magnetic tape by means of a microprocessor and converts the data back into analog audio signals that can be used by the amplifier of a conventional stereo sound system.
• The early 1990s saw the introduction of digital compact cassette (DCC) recorders, which were similar to DAT recorders but could play the older analog tape cassettes in addition to similarly shaped digital cassette.

Conversion of sound to Binary form:

• To make a sound to the binary form, you have to quantify the sound signal form. The more the signal is divided, the more accurate the final signal is obtained, however you cant reach to the real signal ( because extreme space is needed to get the original signal ).
• After quantifying the main analogue signal you can convert the decimal magnitude to the binary form.

How to recover sound from digital form:

• In order to be able to recover the sound again after converting it to digital form you have to consider:

1. The aliasing problem by passing your signal into antialliasing filter which limits the bandwidth of the sound signal to the highest desired one such that one preserves the bulk of the signal and cuts its tail.
2. Then you have to sample this signal with at least fs>= 2 the maximum frequency contained in the signal.
3. As fs increases the reconstruction of the signal becomes easier but it needs higher speed converters.
4. Then after sampling you have to convert it into digital form using an A/D converter with the a conversion speed grater than or equal to the sampling frequency.
5. After that to recover the signal again you have to use the appropriate speed and resolution.
6. Then you pass the signal into low pass filter to extract the original signal from the back converted waveform.
7. You can use also interpolation after backconvertion.

Sound into numbers:

• In a digital recording system, sound is stored and manipulated as a stream of discrete numbers, each number representing the air pressure at a particular time.
• The numbers are generated by a microphone connected to a circuit called an ANALOG TO DIGITAL CONVERTER, or ADC.
• Each number is called a SAMPLE, and the number of samples taken per second is the SAMPLE RATE.
• Ultimately, the numbers will be converted back into sound by a DIGITAL TO ANALOG CONVERTER or DAC, connected to a loudspeaker.

BINARY DECIMAL
0=0
1=1
10=2
11=3
100=4
1111=15
1111111111111111=65535

• Each digit in a number is called a BIT, so that last number is sixteen bits long in its binary form. If we wrote the second number as 0000000000000001, it would be sixteen bits long and have a value of 1.

Effect of increasing sample rate:

• Even at high sample rates, the output of the system is a series of steps. A Fourier analysis of this would show that everything belonging in the signal would be there along with a healthy dose of the sampling rate and its harmonics.
• The extra junk must be removed with a low pass filter that cuts off a little higher than the highest desired frequency. (An identical filter should be placed before the ADC to prevent aliasing of any unsuspected ultrasonic content, such as radio frequency interference.)

Recording Digital Data:

• Once the waveform is faithfully transformed into bits, it is not easy to record. The major problem is finding a scheme that will record the bits fast enough.
• To record on tape, a very high speed is required to keep the wavelength of a bit at manageable dimensions.
• This is accomplished by moving the head as well as the tape, resulting in a series of short tracks across the tape at a diagonal.
• On a Compact Disc, the bits are microscopic pits burned into the plastic by a laser.The stream of pits spirals just like the groove on a record, but is played from the inside out.
• To read the data, light from a gentler laser is reflected off the surface of the plastic (from the back: the plastic is clear.) into a light detector. The pits disrupt this reflection and yield up the data.
• In either case, the process is helped by avoiding numbers that are hard to detect, like 00001000.
• That example is difficult because it will give just a single very short electrical spike. If some numbers are unusable, a larger maximum (more bits) must be available to allow recording the entire set.
• On tape, twenty bits are used to record each sixteen bit sample, on CDs, twenty-eight bits are used.

The Benefits of Being Digital:

• Digital devices usually require less maintenance than analog equipment. The electrical characteristics of most circuit elements change with time and temperature, and minor changes slowly degrade the performance of analog circuits.
• Digital components either work or don't, and it is much easier to find a chip that has failed entirely than one that is merely 10% off spec.
• Many analog systems are mechanical in nature, and simple wear can soon cause problems. Digital systems have few moving parts, and such parts are usually designed so that a little vibration or speed variation is not important.