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[Elliot Thompson]

Hey guys!

I know sinewaves are used when manufacters spec amplifiers.
 
However, I have no idea what Square, Triangle, & Sawtooth waves are used for.

I was reading this, and, it says a square wave offers 50% Duty Cycle.

Would this be more towards a torture test for the amplifier,
than loading a 2 ohm load, using sinewaves?


Thanks,

[Gareth James]

I don't really know much (read anything) about amp testing but a square wave as im sure you know contains harmonics. The "pureness" of the square shape is dictated by the higher harmonics, the more harmonics the purer it becomes.

I don't know if the fact a pure square wave has 50% duty cycle, (as quoted from that link you gave), would mean the amp would be pushed much harder than just the fundamental as a sine wave... actually reading that linkpage again it says 50% duty cycle is also its average power...

Isn't the average power of a sine wave 70% of its peak (or peak/root 2 or whatever)?

Maybe the extra frequencies add bandwidth and decrease actual continuous power...i don't know, this is just my guess...so ill wait for someone who does know to explain it to us all

JR seems to know lot bout the electronics/electrical side of things....dude??!

Gareth James

[Al Limberg]

Typically they are waveforms employed in analog and some digital I suppose synthesizers.  The old Yamaha P2xxx series amplifiers used to show a screen shot of the amplifiers output with a square wave input.  Of course since slew rate would have to be infinite in order to actually reproduce a square wave one hoped for the most squarish waveform obtainable with an eye toward the 'overshoot' at the corner of the rise.  I think the downside of testing amps with square waves would be that, while sinewaves are harmonicaly pure (and thusly relatively easy to measure in terms of distortion), the square wave is laden with harmonics (and not necessarily pretty ones!).

I'm sure JR or Bob will have alot better input here.

Al

[Jonathan Novick]

Sine waves are ideal test signals because they contain only one component in the frequency domain. This makes filtering easy as well as fast fourier analysis.

A square wave consists of a series of in phase sine waves of the odd harmonics. Each one is reduced in strength by the level of that harmonic. For instance, the third harmonic will be at 1/3 the level, the fifth will be at 1/5th the level. An ideal square wave has vertical sidewalls. Slew rate is measured as dV/dT. Since the dT term is essentially zero, the slew rate of an ideal square wave is infinite. This makes square waves an ideal signal to measure slew rates and transient response.

Burst testing is another way amplifiers are tested. A burst signal is basically a sinewave mixed with a lower frequency square wave. This produces a sine wave with a square wave shaped envelope. This is ideal for testing transient/dynamic intermodulation.

Sawtooth waves are often used as ramp generators in effects units and synthesizers. However, they are almost never used in typical audio test situations.

[Tom Reid]

A good application for square waves is the clock on your computer, memory reads, digital representations of analog input.

At the simple level 0V is off, +5V is on.

An amp can reproduce square waves, my thinking would be the more accurate a square wave can reproduced, the better the amp.

[Alex_C]

Don't get square waves near your speakers! One thing that they are frequently used for is destructive testing [of speaker drivers].

They can also be used for testing the transient response of power amps, as has been explained.

Other than that, I can't really think of any practical uses in live audio for square, triangle, or sawtooth waves.

[John Roberts {JR}]

Elliot Thompson wrote on Sun, 31 July 2005 09:10

Hey guys! I know sinewaves are used when manufacters spec amplifiers.   However, I have no idea what Square, Triangle, & Sawtooth waves are used for. I was reading this, and, it says a square wave offers 50% Duty Cycle. Would this be more towards a torture test for the amplifier, than loading a 2 ohm load, using sinewaves? Thanks,

The classic problem when using test signals for evaluating amplifiers or circuitry is knowing what you're looking for and what deviations are important what less so. Regarding square waves for stressing amplifiers, it will simultaneously pull more power from the transformer and power supply while allowing the output power stage an easier time since there is less voltage drop and power dissipation in output devices to deal with.

Square waves have been used for years as a quick and simple way to asses a signal path. Tilt in the horizontal portion of the waveform will reflect LF cutoff. The shape of the rising and falling edges can indicate other characteristics about the design.

In a properly band limited design the edge of the square wave will show a steep exponential curve slowing near the top. This rise time will be defined by the HF cutoff. Note: the rise time for a 1V and 100V square wave will be the same. If instead this rising edge looks like a straight line just tilted a constant amount from the vertical, that suggests the amplifier or circuit is slew rate limiting and not operating linearly. It's not uncommon for such an amplifier to overshoot, ring, and otherwise distort when the square wave stops rising. This overshoot from a poorly designed circuit should not be confused with "Gibb's" phenomenon which is sometimes seen when a very good digital filter removes just the uppermost harmonics of a square wave. While Gibb's looks a little like ringing it's symmetrical occurring just before and after the rising edge (looks like a HF sine wave superimposed on a square wave because that's pretty much what it is, just the missing component not added).

Saw tooth and asymmetrical waveforms are probably more used in old analog synths and analog control circuits. Back in the good (very) old analog days a sawtooth might be used to drive a VCO to create a sine wave sweep. In Synth's they somewhat mimic the harmonic character of some brass and may be more audible wrt absolute polarity but I am not aware of any significant use in testing as a waveform.

The best future test is probably shaped noise and I could imagine a very precise digitally generated noise waveform to excite a circuit or system and a digital receiver that precisely counts the many ways the waveform it receives varied from the reference original. We have historically used sine waves because they were pretty straightforward to generate and analyse. This analysis routinely ignores things like polarity, time delay, phase shift, etc. A very precise review of how a noise waveform gets altered could be pretty instructive but probably be quite difficult to fully interpret the significance of all the results. Audio already has a long history of design excesses and marketing programs based on poorly understood metrics, since amps are getting more alike these days, it's about time to bring in some new tests to misunderstand.      

JR

[John Roberts {JR}]

alexc wrote on Sun, 31 July 2005 12:36

Don't get square waves near your speakers! One thing that they are frequently used for is destructive testing [of speaker drivers]. They can also be used for testing the transient response of power amps, as has been explained. Other than that, I can't really think of any practical uses in live audio for square, triangle, or sawtooth waves.

I'm not sure what destructive testing might be, thermal speaker testing is done with shaped noise.

I don't find square-waves very useful for listening tests with speakers because of all the harmonic content. There is an audio myth floating around about square waves being more thermally stressful because they are sitting still most of the time. I suspect this myth arose just like most myths to explain the still poorly understood mechanisms surrounding amplifier clipping (it's the gain).

-----

A tone burst is far more useful for evaluating transient response. A square wave will act just like a severely distorted tone. You might be thinking of using a square wave as stimulus for measuring output rate of change (see my other post for discussion of rise time).

JR

[Alex_C]

Quote:

I'm not sure what destructive testing might be, thermal speaker testing is done with shaped noise.

Mechanical destructive testing, according to a speaker-designer friend. No, he doesn't work for a reputable manufacturer (IMNSHO)!

Quote:

A tone burst is far more useful for evaluating transient response.

Yeah, I said slew rate. Stupid spell-checker must have misunderstood me .

Edit: Spelling corrections. Time for bed/more caffeine

[Elliot Thompson]

Thanks for the input guys!

JR wrote on Sun, 31 July 2005 18:56

Regarding square waves for stressing amplifiers, it will simultaneously pull more power from the transformer and power supply while allowing the output power stage an easier time since there is less voltage drop and power dissipation in output devices to deal with.     JR

I swear you explained exactly what happened to
the QSC, when I tried to find out, how low of a tone the
amp will put out, before triggering the protect switch.


I conducted the test around 3:30 am. I first started
with a 4 Hz sinewave with no results. Then I tried
a 4 Hz Square wave and ran the amp straight into clip.

Upon clipping I watched the power indicator flicker
lightly, which didn't happen with the 4 Hertz sinewave.

Seeing that I was feeding only one channel, I could
easily monitor the difference with the channel getting
no signal.


Best Regards,