Background for Comms Media Lab

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Background for Comms Media Lab

• Signals and Signal Processing
• George South
• 16 Jan 2007

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Content

• Frequency - measurement etc
• Frequency content of base band signals
• Modulation
• why and how
• frequency ranges used in UK
• Sampling and resolution
• Streaming

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1. Frequency and bandwidth

• The frequency of a signal is a very important
  characteristic
• State the frequency of the following signal
• Hint work out the period the time for one
  complete cycle
• Frequency number of cycles per second
• (1 / period) Hz

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Frequency calculation - continued

• T period 1 micro second 1 10 - 6 seconds
• F 1 / period 1 10 6 1 MHz

one complete cycle
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Example using CoolEdit

• Open the file say topic4-A.wav
• Note the following
• Sampling rate e.g. 44100 Hz (CD)
• Channels Mono/ Stereo
• Resolution e.g. 16-bit
• Use the zoom into feature to view waveform

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Screen shot of topic4-A.wav
Either work out the frequency from the period or
cheat by using the analyze feature
ZOOM Button
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Screen shot of Analysis of topic4-A.wav
The frequency is 7350 Hz
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Calculating the frequency

• Look at the screen shot for topic4-A.wav
• It is a square wave sampled at 44100 Hz
• There are 6 samples per squarewave
• The sample time is 1 divided by 44100
• There are 6 samples per period of waveform
• Period 6 times 1 divided by 44100
• Frequency 1 / Period 44100 divided by 6
• Frequency 7350 Hz

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2. Frequency content of waveforms

• In the Topic 3 Modems lab you analysed the
  frequency content of a square wave and found it
  contained harmonics
• A 1 kHz square wave has harmonics at 1 kHz, 3
  kHz, 5 kHz, 7 kHz etc
• The following example builds up a square wave
  from pure sine waves

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Square Wave
1 kHz sine wave
1 kHz sine wave plus 3 kHz (third harmonic) with
1/3 amplitude
1 kHz sine wave plus 3kHz, 5 kHz, 7 kHz
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Odd Harmonics up to 11
This is a square wave with harmonics 3,5,7,9,11
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Using the Waveform Generation Package

• A simple Delphi program exists to demonstration
  synthesising a square wave from harmonics
• The program is simply launched by double clicking
  on the Delphi project file squarewave1p.dpr
• All you need to do is to move the brace to edit
  out unwanted harmonics

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3. Modulation

• Modulation is important in many communication
  systems
• Base band signals are simple e.g. Ethernet
• However modulation is vital in order to share
  bandwidth
• Modern systems use complex modulation techniques
  but the basic ideas are still the same

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An example of AM
25 is scaled to represent 250 kHz
1.0 is scaled to represent 10 kHz
The carrier frequency is set to, say 250 kHz and
the modulation frequency is 10 kHz The bandwidth
is 20 kHz
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Amplitude Modulation investigated

• What did the green lines represent on the last
  slide ?
• This tells us about the spectrum or frequency
  content of the AM waveform
• There are three parts

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Amplitude Modulation

• So the 10 kHz modulating frequency has been
  processed to become two frequencies
• The lower frequency is called the lower sideband
  and is (carrier freq mod freq)
• The upper frequency is called the upper sideband
  and is (carrier freq mod freq)

Lower sideband at 240 kHz
Carrier at 250 kHz
Upper sideband at 260 kHz
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An AM station Virgin 1215 kHz 0r 247 metres
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An example of Frequency Modulation
The carrier frequency is set to say 250 kHz and
the modulation frequency is 10 kHz Mod index
5 The bandwidth is 140 kHz
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FM investigated

• What did the black lines represent on the last
  slide ?
• This tells us about the spectrum or frequency
  content of the FM waveform
• There are three parts
• 7 lower sidebands
• The carrier at 250 kHz
• 7 upper sidebands
• The FM spectrum is more complex than the AM
  spectrum and needs much more bandwidth

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Comments on FM

• AM broadcasts are prone to problems with noise
  why ?
• Noise or interference will usually change the
  amplitude of the signal but this is where the
  information is
• FM is better for several reasons
• The information is contained in the frequency
  content not the amplitude
• The information is repeated many times so there
  is redundant information
• Whats the catch ? FM is more complex

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Frequency ranges in UK

• In the UK, AM radio uses frequencies between
  about 180 kHz and 1.6 MHz
• FM radio ranges from 88 to 108 MHz
• TV ranges from 470 to 854 MHz
• Cellular mobile services operate within the
  frequency ranges 872-960 MHz, 1710-1875 MHz and
  1920 - 2170 MHz.

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Selected BBC FM transmitters out of about 100
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An example of QAM
The information is contained in the amplitude and
in the phase shift there are 2 amplitudes and 4
phase shifts
http//www.physics.udel.edu/wwwusers/watson/studen
t_projects/scen167/thosguys/qam.html
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4. Sampling and quantisation

• Which of the following audio waveforms will give
  the best quality?
• Why?
• A - 16 samples but low resolution
• B 16 samples better resolution
• C more samples
• D more samples and better resolution

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Playing the track - Little Lies
This is part of a CD track
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Questions

• Is the previous audio clip running under CD
  conditions ?
• Estimate the bandwidth used by the clip
• Does this correspond to the sampling theorem ?
• Answer Yes Sampling rate 44.1 kHz and
  resolution 16-bits
• Answer Somewhat over 11 kHz
• Answer Yes with 44.1 kHz frequencies up to
  22.05 kHz could be sampled

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Is CD quality sufficient ?

• The answer is yes
• What about 48 kHz and 96 kHz sampling and DVDs ?
• There is a lot of info on this on the internet
• In general musical instrument do produce outputs
  above 30 kHz
• Under ideal studio conditions there will be a
  benefit

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Global System for Mobile Communications

• The international, pan-European operating
  standard for the current generation of digital
  cellular mobile communications. It enables mobile
  phones to be used across national boundaries. In
  the UK this technology operates in the 900 MHz
  and 1800 MHz frequency bands.
• So called 2G technology

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Universal Mobile Telecommunication System

• The next generation of mobile phone technology,
  expected to result in widespread use of video
  phones and access to multimedia information.
• In the UK this technology operates in the 2 GHz
  region

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Overview

• Most transmission systems use some form of data
  compression
• Mobile phones would ideally use 8 kHz sampling
  and 8-bit resolution
• The resulting data rate 64 kbps is too high
• Mobile phones use digital signal processing to
  reduce the data rate required to between
  9.6kbit/s and 4.567kbit/s.

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The need to compress files

• A 640 Mbyte CD plays for about 1 hour
• This is 10 Mbyte/minute or about 1/6 of a
  Mbyte/sec
• The actual value is 44100 times 16 times 2
  1,411,200 bps
• How can the data rate be reduced ?
• Reduce the quality ( mono, 8 kHz, 8-bit)
• compress

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The need to compress files .. continued

• There are three samples below
• An original WAV file
• An compressed version of the WAV file
• A decompressed version of the WAV file

Compressed 336 kByte
Decompressed !!!
WAV original 3600 kByte
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(No Transcript)
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info on 44.1 kHz

• Total number of bits Sample Rates for Digital
  Audio The 'REVISION of AES5 AES recommended
  practice for professional digital audio
  Preferred sampling frequencies for applications
  employing pulse-code modulation' says the
  following'A sampling frequency of 48 kHz is
  recommended for the origination, processing, and
  interchange of audio programs employing
  pulse-code modulation. Recognition is also given
  to the use of a 44,1-kHz sampling frequency
  related to certain consumer digital audio
  applications, the use of a 32-kHz sampling
  frequency for transmission-related applications,
  and the use of a 96-kHz sampling frequency for
  applications requiring a higher bandwidth or more
  relaxed anti-alias filtering.' For digital
  consumer audio audio the sample rate of 44.1 kHz
  was chosen because early digital recording
  systems using PCM adapters recorded on video tape
  (e.g. the Sony PCM 1610 and PCM 1630 machines).
  The sample frequency had to be related to a
  frequency common to both 625 and 525 line video
  systems. (In 625 line 25 Hz (PAL) systems the
  line frequency is 15.625 kHz and 588 out of the
  625 lines are "active" for carrying video
  information. If three samples are recorded per
  line the sample rate is 15.625 x 588/625 x 3
  44.1 kHz. In 525 line 30 Hz systems the line
  frequency is 15.75 kHz and 490 out of the 525
  lines are "active" for carrying video
  information. If three samples are recorded per
  line the sample rate is 15.75 x 490/525 x 3
  44.1 kHz.) Because of the fixed standard for
  the regular audio CD, the standard of 44.1 kHz
  will be around for a long time. A sample rate
  of 48 kHz is the current standard which most
  manufacturers follow (even when the internal
  processing rate is 96k or 192k in order to make
  the internal process faster). The conversion from
  48 kHz to 96 kHz or 192 kHz is much easier than
  the conversion from 44.1 kHz to 48 kHz or vice
  versa. The sample rate of 48 kHz is adopted from
  the entire film, video and broadcast industry and
  also DVD is based on the standard of 48/96 kHz.
  Recording studios will go with the 44.1 kHz
  standard, if the end product is CD and not DVD
  etc. A sample rate of 96 kHz instead of 48 kHz
  has for sure an advantage in audio quality (if
  the source material is adaquate), but the bigger
  advantage is that the latency will be cut into
  half!
• about 24 bits or 3-bytes

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Speech Encoding

• One irony of the advance in mobile phone
  technology is that while potential data rates are
  increasing the data rate required for speech
  transmission is reducing due to advances in
  speech coding. To digitally represent speech
  without coding techniques a digital bit stream of
  64kbit/s is required this gives good dynamic
  range and adequate bandwidth. This is rather a
  high bit rate, therefore all mobile phones use
  techniques such as linear predictive coding with
  regular pulse or algebraic code excitation to
  reduce the data rate required to between
  9.6kbit/s and 4.567kbit/s.
• It is this lower rate digital data that is
  encoded onto the radio frequency (RF) signal
  through a process called modulation. Two
  modulation methods you are probably familiar with
  are amplitude modulation AM and frequency
  modulation FM. The modulation schemes used for
  digital radios are more complex exploiting
  mathematical properties of the signals in ways
  that are not possible with normal broadcast
  radio. Once modulated the RF signal is then
  amplified and transmitted.