1. Multimedia Digital Audio
Lecture # 6
Razia Nisar Noorani

2. Outline Audio Basic Digital Audio Concepts Streaming Audio
Web Audio Formats

3. Sound Sound comprises the spoken word, voices, music and even noise.
It is a complex relationship involving a vibrating object (sound source), a transmission medium (usually air), a receiver (ear) and a perceptor (brain). Example banging drum.
As the sound vibrates it bumps into molecules of the surrounding medium causing pressure waves to travel away from the source in all directions

4. Sound
So, Sound are rapid vibrations that are transmitted as variations in air pressure.

5. Waveforms Sound waves are manifest as waveforms
A waveform that repeats itself at regular intervals is called a periodic waveform
Waveforms that do not exhibit regularity are called noise
The unit of regularity is called a cycle
This is known as Hertz (or Hz) after Heinrich Hertz
One cycle = 1 Hz
Sometimes written as kHz or kiloHertz (1 kHz = 1000 Hz)

6. Waveforms
Time for one cycle
distance
along wave
Cycle

7. The characteristics of sound waves
Sound is described in terms of two characteristics:
Frequency
Amplitude (or loudness)
the rate at which sound is measured
Number of cycles per second or Hertz (Hz)
Determines the pitch of the sound as heard by our ears
The higher frequency, the clearer and sharper the soundthe higher pitch of sound

8. The characteristics of sound waves
Amplitude
Sound’s intensity or loudness
The louder the sound, the larger amplitude.
In addition, all sounds have a duration and successive musical sounds is called rhythm

9. The characteristics of sound waves
Time for one cycle
Amplitude
pitch
distance
along wave
Cycle

10. Example waveforms
Piano
Pan flute
Snare drum

11. Capture and playback of digital audio
Air pressure
variations
DAC
Converts back into voltage
Digital to
Analogue
Converter
Captured via
microphone
Air pressure
variations
ADC
Signal is
converted into
binary (discrete form)
Analogue
to Digital
Converter

12. The Analogue to Digital Converter (ADC)
An ADC is a device that converts analogue signals into digital signals
An analogue signal is a continuous value
It can have any single value on an infinite scale
A digital signal is a discrete value
It has a finite value (usually an integer)
An ADC is synchronised to some clock

13. The Analogue to Digital Converter (ADC)
It will monitor the continuous analogue signal at a set rate and convert what it sees into a discrete value at that specific moment in time
The process to convert the analogue to digital sound is called Sampling. Use PCM (Pulse Code Modulation)

14. Digital sampling Sampling frequency
SCA Introduction to Multimedia
22 April 2017
Digital sampling Sampling frequency
Lecture 7

15. Digital sampling Sampling frequency
SCA Introduction to Multimedia
22 April 2017
Digital sampling Sampling frequency
Lecture 7

16. Sampling Two parameters: Sampling Rate Frequency of sampling
Measure in Hertz
The higher sampling rate, higher quality sound but size storage is big.
Standard Sampling rate:
KHz for CD Audio
KHz
KHz for spoken
KHz for audio effect

17. Sampling
Size sample
The resolution of a sample is the number of bits it uses to store a given amplitude value, e.g.
8 bits (256 different values)
16 bits (65536 different values)
A higher resolution will give higher quality but will require more memory (or disk storage)

18. Quantisation
Samples are usually represented the audio sample as a integers(discrete number) or digital

19. Basic Terms Term Definition Bit Binary value 0 or 1 Kilobit (Kb)
1,000 bits (approx.)
Byte
8 bits
Kilobyte (KB)
1,000 bytes
Megabyte (MB)
1,000,000 bytes
Gigabyte (GB)
1,000,000,000 bytes
Kbps
Kilobits per second (1,000 bits in a second)
KB/sec
Kilobytes per second (1,000 bytes in a second)

20. Analog vs. Digital
Analog: A signal that has a continuous nature rather than a pulsed or discrete nature.
Note: Electrical or physical analogies, such as continuously varying voltages, frequencies, or phases, may be used as analog signals.
Digital: A signal in which discrete steps are used to represent information.

21. Digital Signal
Use Pulse Code Modulation (PCM) to represent an audio signal by digital data.

22. ADC & DAC
Figure 4.3 Conversion from Analog to Digital requires an Analog-to-Digital Converter
Figure 4.4 Conversion from Digital to Analog requires a Digital-to-Analog Converter

23. Basic Digital Audio Concepts
Sampling rate
Number of sample taken of a signal in a given time (usually one second)
Bit depth
Describes the accuracy of the audio data
Channels
E.g. Stereo has two channels
Bit rate
Measured in kilobits per second (Kbps) is a function of the bit depth and sampling rate

24. Sampling rate
The more sample taken per second, the higher the accuracy.
Typically measured in kilohertz (KHz).
CD audio has 44,100 samples per second (44.1KHz).
8 KHz produces lower quality radio sound.
Standard sampling rates include “8KHz”, “11.025KHz”, …
The high-end 96K is used in DVD, but is not applicable to the Web.

25. Sampling Rate
demo

26. Bit depth Also called “sampling resolution” or “word length”.
The more bits, the better is the quality of the audio (and a larger file of course).
Common bit depths are 8-bit (telephone like), 16-bit (CD quality), and 20, 24, 32, 48-bit depths.
How many signal can a 8-bit and a 16-bit data represent?  

27. Quantization
demo

28. Channels Audio file can support one to six channels of audio formats.
Mono – one channel
Stereo – two channels
Some others – three, four channels.
Six channels – 5.1-channel surround sound.
More multi-channel formats announce in the coming years.

29. Channel Examples

30. Bit rate
Audio files are measured in terms of bit rate which is measured in kilobits per second (Kbps).
It can be calculated by dividing the file size by the time (in second) to play the audio clip.
E.g. 3Mb file play in 30 seconds
3000k / 30 = 100kbps.
Quality at different compression rates

31. Optimizing for the Web Length of the audio clip Number of channels
Keep the audio clip as short as possible.
E.g. just keep the most sweetest part of your greeting.
Number of channels
A mono audio file is halved the space of a stereo file.
Depends on your actual needs.

32. Optimizing for the Web Bit depth Sampling rate
Audio file on the Web are usually 8-bits.
Half the size of a 16-bit file.
Sampling rate
Half the sampling rate will also halve the space needed.
Voice only audio file can be reduced to 8KHz.
22 KHz music clips are acceptable.
Putting all things together: Mono, 8-bit, 22KHz, MP3 compression.

33. Calculate audio size 8-bit mono: seconds x KHz
16-bit mono: (seconds x KHz) x 2
8-bit stereo: (seconds x KHz) x 2
16-bit stereo: ([seconds x KHz] x 2)x2
E.g. the file size of 30 seconds of 16-bit, 11KHz mono audio:
(30 x 11) x 2 = 660K.

34. Streaming Audio What is it? Advantages: Disadvantages: Reference
Play almost immediately after the request, continues playing the transferring data.
Advantages:
Address the problem of long download time.
Control distribution and protect copyright, because the user cannot get a copy of the file.
Disadvantages:
Sound quality may be be affected by low speed or unstable Internet connection.
Reference

35. What is Streaming?

36. How does it work? Streaming audio
Packet are sent to a buffer on the receiving
Computer, the RealPlayer will play the sound
File when buffer full
Web browser
Web server
RealAudio
Server
Web browser request
a RealAudio from
the web server

37. Quality Comparison

38. Web Audio Formats WAV/AIFF (.wav/.aif/.aiff)
The Waveform Audio File format (.wav) was developed by Microsoft, supports arbitrary sampling rates and bit depths.
The Audio Interchange File format (.aif, .aiff) was developed for Macintosh platform.
They are less likely used on the Web, because people use “mp3” or “streaming”.

39. WAV/AIFF Try the Bell-lab synthesis link.
WAV/AIFF
Try the Bell-lab synthesis link.
Select the
Audio format

40. MP3 (.mp3)
Able to maintain excellent sound quality at very small file sizes.
The compression reduces an audio file to one-tenth of its original size.
E.g. 40MB file  3.5MB
MP3 is actually MPEG-1 Layer-III
Good for distribution of HQ audio.
Demo:

41. What is MP3 digital encoding?
MP3 is actually the file extension for MPEG, audio layer 3. Layer 3 is one of three coding schemes (layer 1, layer 2, and layer 3) for the compression of audio signals. Layer 3 uses perceptual audio coding and psychoacoustic compression to remove all superfluous information. (More specifically, it removes the redundant and irrelevant parts of a sound signal--the stuff the human ear doesn't hear anyway). It also adds a MDCT (Modified Discrete Cosine Transform) that implements a filter bank, increasing the frequency resolution 18 times higher than that of layer 2.

42. MP3 Players

43. Apple QuickTime Audio (.mov)
QuickTime is a well-known video format, but it can create audio-only movies.
QuickTime is a container format, which is able to store still images, movie formats, …
Excellent compression, true streaming
Netscape and IE have Plug-in now.
Quicktime : demo

44. MIDI (.mid/.midi)
MIDI stands for “Musical Instrument Digital Interface” which is developed for electronic musical instruments.
MIDI files are very compact and very good for low-bandwidth delivery.
Instruments are “piano, drums, bass, orchestral strings, …”
It is very attractive for adding MIDI file to your website with very little download time.
Demo:

45. MIDI

46. RealMedia/RealAudio (.rm/.ra)
RealAudio is a server-based streaming audio format.
The RealServer responses to the requests and delivers the streaming packets, including the bandwidth negotiation.
A robust RealServer can support thousands of simultaneous listeners.
Good for continuous-playing audio and live broadcasts to a large group of people.
Example: RTHK Radio

47. RealMedia/RealAudio

48. Windows Media (.wma/.asf)
Windows Media is a streaming system.
It wraps all media elements into a Active Streaming File (.asf).
Audio may be saved as non-streaming Windows Media Audio format (.wma).
Good for continuous-play audio.
The encoder and player is Free, shipped with Windows OS.

49. Windows Media

50. Choosing an audio format
Audio Needs
Suggested formats
Short voice greetings
WAV, AIFF, MP3
News broadcasts
Streaming solutions (RealAudio, Windows Media)
Background music
MIDI, WAV
Music samples for some audience
MP3 or QuickTime
Radio-style or Live broadcasting
RealMedia System
Musical E-greeting card

51. Add Audio to your Web Page
A simple link
<A HREF=“audio/song.wav”>Play the song </A>
<A HREF=“music.mp3”><IMG SRC=“buttons/play.gif”></A>
Background Sound
<BGSOUND SRC=“audio/song.mid” LOOP=3>
Link to RealMedia
<A HREF=“song.ram”>Link to the song</A>
pnm://domainname.com/song.rm

52. Digital audio editing software
One of the most powerful and professional PC-based packages is a tool called Sound Forge

53. Editing Digital Audio Trimming Splicing and assembly
Volume adjustments
Format conversion
Resampling or downsampling
Fade ins and fade outs
Equalization
Time Stretching
Digital Signal processing
Reversing Sounds

54. Editing Digital Audio Simple audio editing software allows:
Recording of digital audio segments
Trimming
Splicing and assembly
Volume adjustments of the entire segment
Reversing Sounds
Copy, cut, paste and delete segments of digital audio
Others audio editing software:
COOL Edit Pro
Gold Wave
PROSONIQ SonicWORX
Samplitude Studio

55. How audio can be used effectively
Examples of uses of audio:
Cautions and warnings
It is a good medium for alerting users to critical information. Some uses include:
Sounding an alarm when a limit is reached
Alerting users when data is entered incorrectly
Music and Sound Effects
These make multimedia interaction more real. Some uses include:

56. How audio can be used effectively
Musical background for a video segment
Birds Songs accompanying photographs in biological field training.
Sound-related data.
Some uses include:
Helping mechanics diagnose engine trouble
Training medical students to recognize different breathing sounds

57. How audio can be used effectively
Direct voice communication.
Some uses include:
Leaving a voice message for other users of an application
Consulting with an expert during a troubleshooting procedure.

58. Advantages and Disadvantages of using audio
Sound adds life to any multimedia application and plays important role in effective marketing presentations.
Advantages
Ensure important information is noticed
Add interest
Can communicate more directly than other media

59. Advantages and Disadvantages of using audio
Easily overused
Requires special equipment for quality production
Not as memorable as visual media

60. Summary There are two main types of digital audio
Sampled audio
Captured by sampling an analogue waveform at a set rate
MIDI data
Instructions on how to perform some musical composition
Sampled audio requires more storage space than MIDI information