MTD205: Assuming you are Conducting the online Zoom Meeting with Audio Speaker from your Computer: Audio Technology Assignment, SUSS, Singapore

Question 1

During COVID pandemic control, Zoom is one of the online popular platforms for meetings. Assuming you are conducting the online Zoom meeting with audio speaker from your computer, the audio signal has only fundamental component at the intelligible frequency of 3.4 kHz with 1 V. It passes through the anti-aliasing filter before goes through the 12 bits 2’s complement ADC inside the computer’s audio DSP chipset to process the audio signal fidelity.
The processed audio signal with the same 1 V goes through an amplifier inside the audio speaker with a volume gain of 2 and produces an additional 3rd and 5 th harmonics at 0.05 V and0.025 V respectively.

(a) Execute the calculation of sampling frequency and hence the bandwidth needed for the anti-aliasing filter. Plot the ideal bandwidth spectrum.

(b) Analyse the quantization level that is required to cover the full voltage swing of 3 V peak-to-peak of the audio input signal. Hence, obtain the quantization error range with the rounding rule being used in the conversion process.

(c) Analyse ADC binary representation for an instantaneous audio input signal of -1.4 V.

(d) Describe the quality of the audio speaker performance by calculating its total harmonic distortion (THD) in percentage. Hence draw the amplitude spectrum of the amplifier at the output.

Question 2

(a) A music song has three signal components with frequency of 3 kHz, 7 kHz, and 15 kHz with amplitude of 5 V, 3 V and 1 V peaks respectively.

(i) Describe the frequency spectrum for the music song.

(ii) The music passes through a multiplicative mixer and produces beat frequencies among each other. Explain the mixing process by calculating the beat frequencies. Ignore same frequency squared mixing.

(iii) Analyse through illustration on the amplitude spectrum of the output of the multiplicative mixer in Question 2a(ii). Hence indicate the beat frequencies in the plot.

(b) The LEO satellite operating at Very High Frequency (VHF) is used for audio transmission from aircraft to air controller communications. The amplitude modulation (AM) technology is used where the audio modulating signal is given as 𝑏(𝑡) = 1 + 6 cos(8000𝜋𝑡) V while the carrier is given as 𝑝(𝑡) = 4 cos(80000000𝜋𝑡) + 4 cos(120000000𝜋𝑡) V.

(i) Derive and analyse through illustration on the frequency spectrum of the spatially modulated signal.

(ii) Determine the receiver carrier and the bandwidth of the recovering filter so as to recover the modulating signal

Question 3

(a) A DVD sampling rate is 96 kHz and 32 bits coding are employed to record 600 minutes of the two-channel stereo uncompressed song. Execute the calculation to analyse the number of DVDs is required to store the songs assuming a DVD size is 4.7 GB.

(b) Discuss the working principle of lossy compression. Analyse the improvement in capacity storage in Question 3(a) using MPEG1.

(c) MIDI protocol is used to demonstrate the application of audio technology in multimedia. A set of MIDI subroutines is being attached below. They can be used in a C-language program to control MIDI devices. The subroutine parameters can be specified in the MIDI specification in Appendix I and all parameter values must be in hexadecimal format. Please use the subroutines to do the following operations.

(i) System Start

(ii) System to select song number 10 at starting point at a count of 500

(iii) Channel 12 plays note middle C for 2000 ms at maximum volume and then
silent.

(iv) Channel 13 plays note at 8372 Hz for 4000 ms at maximum volume and then middle volume following formula to obtain pitch value given as P = 69 + 12log2(f/440).

(v) Channel 14 plays note at 622.3 Hz for 1000 ms at maximum loudness and the quarter maximum loudness following formula to obtain pitch value given as P = 69 + 12log2(f/440).

Question 4

(a) Discuss and illustrate the effect of early reflection and critical distance.

(b) Analyse the intensity difference (ID) reduction in dB at the receiver for a direct sound signal and reflected sound that travelled 10 m and 20 m respectively. Assume uniform media transmission. Comments on the effect when the direct and reflected sound are totally out of phase with each other and equally in strength? Name one such application.

(c) Obtain the Inter-aural delay (ITD) of sound waves arriving at the human at 30 degrees from a right audio speaker from a computer with respect to the centre. Assume the distance between two ears is 35 cm. Analyse the differentiating capability of the ear for the calculated ITD. Assume the speed of the sound waves in the air is 330 m/s.

(d) Infinite impulse response (IIR) single delay tap filter acting as a reverberation filter is used to produce various reflection sounds. Its output response is given as 𝑦(𝑛) = 𝑥(𝑛 − 1) − 𝑔 ∗ 𝑦(𝑛 − 3) where 𝑥(𝑛) is the input power level sequence of {10, 10, 8, 8, -1, -1,-6,-6,2…0} and 𝑔 is the coefficient that determines the decay level of reverberation with a value of 0.5. Draw the IIR filter and determine the output at 10 seconds. Assume input and output remains 0 when 𝑛 = 0 and n is an integer