Lecture 5-Recording Formats

Question 1

History of audio recording and reproduction:

Answer 1

1857-monophonic recordings 
Phonautograph 
1878-mechanical cylinder phonograph 
1940's-Vinyl 
1928-tape recorder 
4-track and 8-track cartridges and compact cassette.
1980's-CD

Question 2

Analog waveforms are…

Answer 2

Continuous
Records have continuous grooves relating to measured sound intensity.
Analog tape uses recorded fluctuation in the magnetic particles on the tape corresponding to sound intensity.

Question 3

What actually makes a good approximation?

Answer 3

We need at least twice the number of sample points per second as the highest frequency in a signal. If we consider the hearing range is up to 20kHz we need 40,000 sample points every second.

Question 4

What is the Nyquist Sampling Theorem?

Answer 4

The sampling frequency should be at least twice the highest frequency contained in the signal.

Question 5

Define aliasing:

Answer 5

Signals with frequencies higher than half the sampling rate will not be accurately reconstructed (the ADC will think they are at a low frequency). We need to ensure that there are NO signals above half the sampling rate in our audio. We use an anti-aliasing filter with a cutoff at fs/2.

Question 6

What is BITS?

Answer 6

Binary digits,
1 bit = 2 levels                           3 bits = 8 levels
0
1
2 bits = 4 levels 
01 
10
00
11

Question 7

Bit Depth

Answer 7

By increasing the bits, the number of possible values grows exponentially. The greater the bit depth, the greater the dynamic range… each extra bit > 6dB increase in dynamic range > less noise.
CD quality is 44.1kHz, 16 Bit
Frequencies up to 22050 with a 96dB dynamic range.