This week in our lecture we went through a Powerpoint which was about Digital Processing. I collected a copy of the Powerpoint on to my pen drive so that I can go through it later in my own time and read it. We also had a multiple choice question paper today.
To start the day off we discussed a typical digital signal processing system. We discussed the steps it takes between recording sound and getting in into an electrical format, editing it, and then getting it back out again. Below is a diagram that I created showing the process.
The steps.
1. The signal is passed in via a microphone or other recording equipment.
2. The recording is then converted from analogue to digital (into binary numbers).
3. Editing is then done to the digital copy. E.g filtering, pitch warp, echo etc.
4. It is then changed from digital back into analogue.
5. It is then smoothed out.
6. The recording is passed back out edited.
The system cannot understand analogue signal so that is why they must be converted first and naturally we don't understand binary so must be converted back again.
Next we spoke about why we would use digital processing, and the three main reasons are:
Precision
Robustness
Flexibility
Precision: The precision of the Digital Signal Processing System is, in theory, only limited only by the conversion process at input and out put (analogue to digital and digital to Analogue).
In practice, sampling rate (sampling frequency) and word length restrictions (number of bits) modify this.
However if the operating speed and word length of the modern digital logic is increased, this allows more areas of application.
However the increase operating speed and word length of modern digital logic is allowing many more areas of application.
Robustness: The Digital Processing Systems robustness is shown clearly when it is compared to the Analogues System. The Digital System is less susceptible to electrical noise (pick-up ) and component tolerance variations due to logic noise margins.
Adjustments for electrical drift and component ageing are essentially removed; This is important for complex systems.
Inappropriate component values can also be avoid with the Digital System. E.g. Very large capacitors or inductors for Very Low Frequency filtering.
Flexibility: Due to the flexibility of the Digital Processing System, its programmability allows it to be upgraded and have its processing operations expanded easily without necessarily incurring large scale hardware changes.
Practical system with desired Time Varying and/or Adaptive characteristics can be constructed.
All of this can only happen if a sound card is working and being used.
We also learned about sampling a signal, this is when the system samples the signal at a time, nT seconds. It then samples the signal ever period after that, T seconds.
The rate that a signal is usually sampled at is double the frequency of the human hearing range. i.e. a signal heard at 10 Hz would be sampled at 20Hz.
Most modern sound cards support a 16 bit word length coding of quantised sample values.
This allows a representation of 2^16 (65536) different signal levels within the input voltage range of the card. Below is an example of this:
Below shows an example of quantising the signal amplitude. It would be rounded to the nearest value as shown. The red squares being the amplitude and the grey showing the quantised samples amplitude.
The Dynamic Range is the ratio of the largest signal amplitude to the smallest. Since a 16 bit word length allows 2^16 (65536) different siganls, the dynamic range (DR) is calculated as
DR = 20log([Voltage range]/[Quantisation step size]) d/b
DR = 20log(2^16) dB
DR = 96 dB
As the human ear has a dynamic range of greater than 120 dB, even “CD quality” reproduction has some compromise.
After the lecture we went to the Lab where we all took part in a test to assess how much we have learned in the last 2 weeks. I feel that I was able to answer most of the question but a few did have me stumped. Once I get my result back I will know what sections I will need to revise more thoroughly. I will not be too disappointed with my mark as this will be a good chance to assess what needs to be addressed now before its too late.
During this week I plan to search for links to help with the previous weeks work that help me to understand and possible but them in those weeks blogs.
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