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 GCSE     Signal     >Bandwidth<     Communication     Distortion     Noise     Waves

Signal Bandwidth

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GCSE    Signals    Questions 0 to 18   -->  View All

Analogue Bandwidth - Everyone

This is the range of frequencies where the output voltage is at least 70% ( 1 / √2 ) of the maximum output voltage.

Bandwidth is also the capacity of a circuit or channel to carry information. More bandwidth = more information.

Measured in Hertz / Kilohertz / Megahertz / Gigahertz

Voltage Bandwidth

• Voltage Bandwidth is the range of frequencies over which the output voltage is at least 0.7 times the maximum output voltage.
• In the image below, the 70% output voltage bandwidth goes from 2 or 3 Hz up to about 20 000Hz.
• The lower cut-off frequency is about 2 or 3 Hz.
• The upper cut-off frequency is about 20 000 Hz.
• This is a 20 kHz (19998 Hz) bandwidth. The red arrow heads show this.
• This would be typical for an audio amplifier with a good bass response.
• Human hearing works for frequencies between 20 and 20 000 Hz.
• AM radio limits the bandwidth to 100 - 4000 Hz.
• Land-line phones limit the bandwidth to 300 - 3000 Hz. This is the minimum for intelligible speech.
• Bandwidth tells you the frequency range over which a circuit or transmission medium will operate.
• Bandwidth is measured in Hz, kHz, MHz, GHz, etc.

• Normal human hearing covers the range from 20 to 20000 Hz. The upper limit drops with age and damage due to exposure to very loud sounds.
• AM radio limits the bandwidth to 4000 Hz. This allows more stations to share the radio band but the sound quality is less good.
• Old-style telephones limit the bandwidth to 3000 Hz. This is just sufficient for intelligible speech. Mobile phones do a bit better.

Power Bandwidth - AS and A Level

• Power Bandwidth is the range of frequencies over which the output power is at least 50% of the maximum output power.
• On the Y Axis the Power is 50%.

Digital Bandwidth

Measured in bits per second.

Example: An Internet connection.

• using a modem 56kb/s is possible - this allows text, poor quality audio and slow image downloads.
• using ADSL Broadband, the bandwidth is between 512 Kb/s to 64Mb/s - this allows good quality audio, fast image downloads and full screen video.
• faster versions of ADSL up to 100Mb/s are being rolled out (2011) - this will allow full screen high quality video.
• Note that b = bit and B = Byte. These are frequently confused.

Data Compression

Data compression can be used to make better use of the available bandwidth. There are techniques for removing information that the ear would not notice.

There is an audio compression technique which discards some of the voice transmission and repeats samples to fill the gaps caused by the discarding. This is quite intelligible but sounds odd and unnatural. For example some "ssss" or "t" sounds get exagerated or could go missing. Skype or facetime over slow connections might show these effects.

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