AV Spec Check Guide

Confounded by the weird terms in the spec sheets of your AV devices? Here's a guide that will remove much of the mystery.


Christmas has just gone by, presents have been opened, and what do you find? A brand new LCD TV, or a swanky new stereo system that you can perch proudly in your living room. But before you can start enjoying your cool new AV gadgets to the fullest, let’s do a quick run through their specs. This way we will understand what they mean, and also what they don’t...

Audio Specs
Frequency Response
This is a basic spec but an important one. It applies both to amp and speaker. It’s a bit more vital for amps, as the sound needs to be amplified correctly for the speaker to do anything at all. It is measured in Hertz, actually a Hertz range. Where did this all this technical jargon come from, especially when everyone is in a party mood?

A typical frequency response curve showing levels for different frequencies

The human ear can hear only a range of available sound frequencies (20 Hz-20 KHz). Thus the amp should be able to reproduce the original song within this range, but that’s not all. If it was, all amps would sound the same, but they don’t, so it's clear the amps produce certain sound frequencies in slightly different amounts. Some amps have louder mid frequencies, while others have louder bass. In the frequency response rating a number in dB units (e.g. +/-3 dB) tells us that the sound might be louder (or softer) by an amount of 3 dB at certain parts of the response curve.

THD
THD (Total Harmonic Distortion) is another rating that says a lot in the fine print, though one can never really judge the actuality of this rating. It stands for Total Harmonic Distortion. As the name suggests, it talks about distortion: a bad word in Hi-Fi parlance. For the curious, harmonic distortion involves extra components in the output signal, that are not originally in your song. This extra material is not random, it appears in a proper structure at the output, in whole multiples of frequencies in your original song.

Another thing inherent in audio gear is noise. This noise is measured and given to you, and mostly the specification reads THD+N, which is a combined reading of all the unwanted stuff your amp produces. Lower is better: a figure of below 0.01% is required.

SNR
This is a detailed spec, the measurement of which varies among products. It stands for Signal to Noise ratio. What does it mean? It basically talks about how much noise is in your system, in an indirect way. Not to be confused with the N in the THD + N, SNR measures the noise level without any input, and compares that to a reference output level. The ratio between the two levels is the SNR. It is rated as a simple dB value; e.g. 90 dB is a common rating. The higher the better. Manufacturers are expected to mention their reference level for checking, but few do.

Sensitivity
This is a straightforward measurement. It’s the amount of volume the speaker can deliver, in terms of dB SPL (an acoustic unit to measure volume in open spaces) right in front of the speaker. The power is limited to one watt, and the distance is to one meter. The higher the value, the better, as that means the speaker can go louder with lesser rotation of the volume knob eastwards. In other words, less power is required to attain a certain level of music.

Output Power
Do I need to explain this? Just for the record, it is the maximum power an amp can deliver, and the maximum power a speaker can draw. It is measured in RMS Watts. There can be a huge number in the specs, though it must be known that on an average, home amps more or less operate with 1 watt. If you have high sensitivity speakers, 8 watts is more than enough for a bedroom.

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Video Specs
Screen size/ Resolution
The aspect ratio, screen size and resolution are the first things you need to check. These days all products mostly have similar ratings, according to their size. The sizes available are from 19 inch to 60 inch for LCDs, besides the odd 104 incher. Plasmas start at 42 inch and go for a couple of ratings more.

The resolution stated is the native resolution, and it means the number of pixels that the screen is made up of. So 1920 x 1080 means the number of pixels horizontally and vertically, in that order. Thus the second number also means the number of lines that your image will be made up of, as digital video, be it broadcast or recorded, is broken up into lines. HD ready is 1366 x 768, while full HD is 1920 x 1080.

Different screen sizes and aspect ratios

Brightness / Contrast
This is what marketers and brands use as weapons these days, and one must not be confused by the crazy numbers and wide ranges quoted. Brightness is measured physically as the amount of luminance. The physical unit of light is used (Candela per square meter), so what you learn is simply how bright the backlight can be. About 600 Cd/m2 is considered good; the higher the better.

Contrast is expressed as a ratio of blackest black to whitest white, and this is frankly a load of crap. Some also talk about dynamic contrast ratio, which often reaches ridiculous values. The truth is that a value of 1500:1 is very high, and very good, as long it is measured correctly. What manufacturers do is measure the black when the TV is off, while others look at only parts of the screen to measure white.

An example of material used to check contrast


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