Dynamic Range

Dynamic Range


This is a measure of the range of signals that a sound card can handle. It's important because while you are trying to listen to a weak signal there may be other signals in the band pass that are huge, and one does not want those huge signals from keeping us from listening to that weak signal.

If a signal exceeds the range that your sound card can handle (overloading) then we see signals all over the place that are really not there, these signals can often wipe out signals that we do want.

It's a little complicated and a visual display will help to understand it, so I will be looking for an illustration.

But dynamic range involves more than just than the ratio of signals a sound card can receive (bits of resolution) it also involves matching the gain of the radio to the voltage range of the sound card.

Lets say your SoftRock can put out a signal that is is 4V peak to peak or 1.4V RMS that is +16dB and you have a sound card with 100dB range so theoretically you could also hear a signal at -84db (not so good) but you really won' experience that why?

First your sound card most likely can not handle a signal so large so you will overload it, so you need to adjust the gain of the radio by 6 dB so the 10dB input to the sound and does not cause an overload, those dB don't just disappear, your card still has a 100dB range so now your radio works from +10 dB to -90dB still lousy but better.

So what happens if the radios gain is increased by 10dB the bottom goes down by 10dB to -94 dB but your upper end suffers by 10dB, remember the total range is 100dB so now your radio can hear a signal that is -94 and overloads past +6dB, that is still pretty deaf. So we increase the gain so it can hear a -120 dB signal but then our top end is -20dB, that is where the SoftRocks are, they can hear pretty good but can't handle large signals without outside help.

What would happen if you had a shoddy 16 bit card? your dynamic range might only be 80 dB so you have a range of -20dB to -100dB, weak signals (-110dB) are not there, on a 24 bit card with 100 dB of dynamic range that signal is fine and clear.

By adjusting the radio's gain we can maximize the performance of the radio, on 80M the average noise is about -100dB so what is the point of having the radio be capable of receiving a -120dB signal? none, we are wasting our dynamic range, if we tweak the radio so it has enough gain then we could receive from -103 dB to -3dB and we don't loose any signals, we lost nothing on the bottom end but gained 17dB on the top end.

Moral of the story, get a 24 bit card they have a larger dynamic range which translates into hearing weaker signals while tolerating strong signals.

A Delta-44 has 99 dB of dynamic range
A Edirol FA66 has 109 dB of dynamic range
The Atlas/Ozy/Janus has 111 dB of dynamic range

More details later and some pictures.

Frequency Response

A sound card that samples at 48KHz should be able to look at signals as high as 24KHz, a card that samples at 192KHz should be able to see signals as high as 96KHz.

Sadly some cards do not follow that rule but instead have fixed filters in front of the card so even if you go to the high sampling speed the bandwidth of the card is limited to as an example 20KHz, such a card would not be a good card for SDR work.

Always check the manufacturer's sight for data to see what the front end bandwidth is for the different rates, it should be 1/2 the sampling speed.

Number of Bits

Number of bits:

The desired number is 24 bits, a 24 bit card has theoretically 16 times the resolution, and the dynamic range compared to a 16 bit card, and therefore you will have a better receiver, but watch out no one really makes a true 24 bit card, they lie through their teeth on this one. If we truly had a 24 bit card we would have an earth shaking receiver, but we are not there yet.

That does not mean that it doesn't make a difference because it does, different manufactures have varying amounts on how close they get to 24 bits, some are only 16 bits of usable resolution some are as high as 20 bits, you want the 20 bit card.

Here is a list of really good cards that perform rather well, as expected they are rather expensive;

Delta-44 and Delta-66 it cost about $125 excellent PCI card for your home computer. It has 4 inputs and 4 outputs hence the "44" as part of it's name, with this card you could run two receivers at the same time.

Edirol FA66 it cost about $250 it has even better specs than the Delta, it's an external sound card that uses Firewire to connect to your PC so it can be used with laptops also.

Lynx professional it cost about $1000, it's about the best commercial sound card available now, it's a external sound card with Firewire connection.

HPSDR.org group Atlas/Ozy/Janus its about $350 for the motherboard and the two cards, it's quality is about the theoretical limit using present technology, it's a set of boards with no case so you are on your own but the performance is breathtaking.

There are several other high priced but with decent performance but I don't have experience or know anyone that does. With the rest the price varies quite a bit but they vary in quality all over the place. More money in this case does not mean better performance, some of the expensive consumer cards can be dogs for SDR use why? Many have excessive noise beyond the human hearing, good enough for audio but not so good for SDR work.There are some good performing cards for low cost to be had, as an example;

Sound Blaster Live 24

Unfortunately this card is not manufactured any more but you can buy them on eBay for about $10 to $20, the card performs quite well getting close to the performance of a Delta-44 but at a minuscule price. The internal Aux input is even quieter than the outside connector. There are some peculiarities with this card, the two samples do not occur at the same time, very common with Sound Blaster cards and other inexpensive cards. Fortunately the software for SDR use for the most part compensate for this oddness. A indication that your software is not compensating or you have not turned on the feature is that you get images of signals on both sides of the band-pass.

Some of it's higher price siblings do not fare so well a very popular card the SB Audigy 2 SZ has some issues with noise specially on the output side, the noise is outside of the audio range so most don't care or even notice it, but our transmitters will send that noise all over the place.

Resolution

Resolution


A cards resolution has to do with how accurate is the card at converting a signal to a unique digital value.

On most 24 bit cards the last 4 to 8 bits is dominated by noise, so although you may have 24 bit A/D converter in your card if fed by a precise voltage at some low level of voltage the readings start wandering all over the place and the result are meaningless.

So what you are really interested in is what is the real accuracy of the card? On a good card you may get as high as 20 bits of accuracy, on a modest card you might get 18 bits and on a so-so card you might get 16 bits of accuracy. Yet 16 bits of accuracy is better than a 16 bit card that only get 14 bits of accuracy. 16 bit cards suffer from the same problem but not as great an extent. So the bottom line is that a medium quality 24 bit card is way better that a great 16 bit card.

Always shoot for 24 bit cards when you can afford it, even if on a modest budget there are inexpensive 24 bit cards that are fairly good such as the Sound Blaster 24 PCI.

What has this to do with a receiver? When your signal is way down in level you want to make sure that signal is not in that bottom area where the readings are worthless, a poor card will give you a muddy sounding signal while a decent card with the same signal will be clear and readable. Which one do you want?

Sampling Rates

Sampling Rate

The sampling rate establishes how wide a frequency span you can see at a time, Nyqist established that the sampling rate must be at least twice the frequency of interest. For 48Khz sampling the computer can see a 24KHz signal, but since we are sampling two signals we actually can see + and - 24KHz for a total of 48KHz of signals. Taking that further if we have a high quality card that can sample at 192KHz then we can see 192KHz segment of the band at one time, not bad for a $11 radio.

The beauty of this is that you engage a second set of sense when using your radio, your sight, you can look a a fair size portion of a band at a time and see where the signals are, and even what kind of signals the are, it a wonderful experience that can get you hooked in no time.

A image of my computer desktop running PowerSDR software, with a SoftRock40 looking at a portion of the CW band

Note the CW signals rising above the noise floor of the receiver.