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→Design of Your Tests
[[Category:SPO600 Labs]]{{Admon/lab|Purpose of this Lab|In this lab, you will investigate the impact of different algorithms which produce the same effect. You will test and select one of three algorithms for adjusting the volume of PCM audio samples based on benchmarking.}}
== Lab 4 ==
=== Background ===
* Digital sound is typically represented, uncompressed, as signed 16-bit integer signal samples. There is one stream are two streams of samples , one each for the left and right stereo channels, at typical sample rates of 44.1 or 48 thousand samples per secondper channel, for a total of 88.2 or 96 thousand samples per second (kHz). Since there are 16 bits (2 bytes) per sample, the data rate is 88.2 * 1000 * 2 = 176,400 bytes/second (~172 KiB/sec) or 96 * 1000 * 2 = 192,000 bytes/second (~187.5 KiB/sec).
* To change the volume of sound, each sample can be scaled by a volume factor, in the range of 0.00 (silence) to 1.00 (full volume).
* On a mobile device, the amount of processing required to scale sound will affect battery life.
=== Basic Sound Scale Program ===
Get the files for this lab on one of the ARMv8 AArch64 [[SPO600 Servers]] -- but you can perform the lab wherever you want.
# Unpack the archive <code>/public/spo600-algorithm-selection-lab.tgz</code>
# Examine the <code>vol1.c</code> source code. This program:
## Creates 5,000,000 random "sound samples" in an input a data array (the number of samples is set in the <code>vol.h</code> file).## Scales those samples by the volume factor 0.75 and stores them in an output back to the data array.
## Sums the output array and prints the sum.
# Build and test this file.
#* Does it produce the same output each time?
# Test the performance of this program. '''Adjust the number of samples as necessary to get measurable results'''.
#* How long does it take to run the scaling?
#* How much time is spent scaling the sound samples? Be sure to eliminate the time taken for the non-scaling part of the program (e.g., random sample generation). '''(How will you do this?)'''
#* Do multiple runs take the same time? How much variation do you observe? What is the likely cause of this variation?
#* Is there any difference in the results produced by the various algorithms? How much does numeric accuracy matter in this application?
=== Alternate Approaches ===
The sample program uses the most basic, obvious algorithm for the problem. Let's call this "Algorithm 0", or the "Naive Algorithm". Note that it uses casting between integer and floating-point formats as well as multiplication -- both of which can be [[Expensive|expensive]] operations.
Try these alternate algorithms for scaling the sound samples by modifying copies of <code>vol1.c</code>. Edit the <code>Makefile</code> to build your modified programs as well as the original. Test each approach to see the performance impact:
# Pre-calculate a lookup table (array) of all possible sample values multiplied by the volume factor, and look up each sample in that table to get the scaled values. (You'll have to handle the fact that the input values range from -32768 to +32767, while C arrays accept only a positive index).
# Convert the volume factor 0.75 to a fix-point integer by multiplying by a binary number representing a fixed-point value "1". For example, you could use 0b100000000 (= 256 in decimal) to represent 1.00, and therefore use 0.75 * 256 = 192 for your volume factor. Multiply this fixed-point integer volume factor by each sample, then shift the result to the right the required number of bits after the multiplication (>>8 if you're using 256 as the multiplier).
* You may need to run a very large amount of sample data through the function to be able to detect its performance. Feel free to edit the sample count in <code>vol.h</code> as necessary.
* If you do not use the output from your calculation (e.g., do something with the output array), the compiler may recognize that, and remove the code you're trying to test. Be sure to process the results in some way so that the optimizer preserves the code you want to test. It is a good idea to calculate some sort of verification value to ensure that both approaches generate the same results.
* Be aware of what other tasks the system is handling during your test run, including software running on behalf of other users.
==== Analyzing Results ====
=== Tips ===
{{Admon/tip|Non-Decimal Notation|In this lab, the number prefix 0x indicates a hexadecimal number, and 0b indicates a binary number, in harmony with the C language.}}
{{Admon/tip|Time and Memory Usage of a Program|You can get basic timing information for a program by running <code>time ''programName''</code> -- the output will show the total time taken (real), the amount of CPU time used to run the application (user), and the amount of CPU time used by the operating system on behalf of the application (system).
Another version of the <code>time</code> command, located in <code>/bin/time</code>, gives slightly different information, including maximum resident memory usage: <code>/bin/time ''programName''</code>}}
{{Admon/tip|SOX|If you want to try this with actual sound samples, you can convert a sound file of your choice to raw 16-bit signed integer PCM data using the [http://sox.sourceforge.net/ sox] utility present on most Linux systems and available for a wide range of platforms.}}
