SpeKtrum DX8's range and output?

[pchristy]

To be honest, I've no idea how many packets per second Spektrum systems transmit!

The output frame rate from the receiver has to be in the 50-60 Hz region to maintain compatibility with existing servos, but that doesn't mean that the *transmitted* frame rate cannot be significantly higher. You need to know how many bits are dedicated to each function - including parity bits and checksums, how fast they are transmitted (not necesarily the same as the output frame rate) and how many bits make up a packet! Little, if any, of this information is readily available, as far as I know!

However, holding a wet finger up in the air, and taking a wild guess, I would think the figure would probably be in the region of the low hundreds per second.

[scallybert]

However, when you look at them on a spectrum analyser, a DSM2 channel is about 1MHz wide. A FASST channel is only about 100KHz (and yes, you have to be quick to see it!)

Interesting.

With DSSS the channel width is related to the bit-width of the 'random number' signal that the data signal is multiplied by. (The random number signal is a significantly higher frequency than the data signal.)

I got the impression that with FHSS, the channels were only as wide as the data signal. 100kHz is too wide for that, maybe ?

But to me, counting packet loss is a bit like putting an oil pressure gauge in you car! If you engine is fragile, or your doing a lot of racing, its probably essential! If you have a robust, modern car, it will probably just worry you unnecessarily!

In the early noughties, Suzuki desensitised the oil pressure sensor on the GSX-R750 (etc), as it was doing exactly that. I found this out as my RF900 oil warning light was worrying me...

Packet loss is something that is expected in SS systems, and the systems are designed to cope with it. The only real test is "does it work"?

Ok, but there's perhaps an additional subtlety.

I'd expect FHSS to be designed survive packet loss, as this will occur as the narrow band signal hops into a noisy frequency - and go away as it hops out.

OTOH, I'd expect DSSS to be designed to deal with losing part of the spectrum its wide channel was using; and have a mechanism to reconstitute the data in a packet from reduncancy in that packet. If you lose packet 'n', there's not necessarilly a reason why packet 'n+1' shouldn't have the same problem.

On that basis, packet loss should be less likely - but more worrying - on DSSS than FHSS. (?)

[Perhaps I should point out that I'm a relatively happy Spektrum user.]

[pchristy]

Scallybert,

As you are clearly aware, the precise whereabouts of the actual data in a DSSS signal is a bit vague! The way I try and visualise it is to imagine an analogue system with the information on a sub-carrier that is varying rapidly in frequency! Not a mathematically accurate analogy, but it helps me to visualise what's happening!

Although the Spektrum channel is 1 MHz wide, the data only occupies about one tenth of this - ie 100 KHz! This is one of the ways Spektrum get around the 10 mW / 100 mW conundrum in the regulations! Which 100 Khz it is occupying at any given moment is a function of the spreading code. (Its a bit like quantum physics, and the Heisenberg Uncertainty Principle!)

In that respect, the end result is that the data "hops" around within the 1 MHz wide channel, but only ever occupies about 100 Khz of the available 1 MHz!

In contrast, the FASST systems simply moves the 100 KHz chunk at random around the whole available band.

As to which is the most immune, despite what many say, the jury is still out! In practical terms, there is little, if anything, to choose between them.

My day job is that of a broadcast video editor. In days of old, when we were working on analogue Video Tape Recorders, a drop-out on the tape resulted on a small - but often noticeable - piece of the picture vanishing.

These days, with digital VTRs, the signal is spread over a large part of the tape surface. If there is a drop out, all that happens is that the signal to noise ratio deteriorates for a moment, but the picture remains substantially intact. This is very similar to what happens with a DSSS system. A missing packet will reduce the S/N ratio, but the data should still be recoverable. With a FASST system, the interference will be skipped over quickly enough for it not to be noticeable.

Both are extremely robust systems, and at the end of the day, which you choose comes down to personal preference! There is little - if any - in the way of technical advantage of one over the other!

[pchristy]

If anyone is interested in a totally unbiased tutorial on how spread spectrum works, follow this link:

Tutorial on Spread Spectrum Technology

[barney]

Very interesting reading. Concerning Speccie range checking, Ive compared a DX7 (mine) and 3 x SpeKtrum modules In detail when we first encountered large range differences.
On one occasion we even rebound 3 different Tx's to the same model to eliminate RX, Radio Install variables including the models orientation and positioning.

My DX7 and a Moduled 9X were giving approx 50 paces in range check mode and 150 Paces (line of sight due to the geography) A new JR9X again with module was around 15 paces and about 60 in full range.!

No 3 was returned for checking and on return was still poor in comparision to the others.

Pete as for multiple frame dropping, it will help spot a poor install or other limiting factor that you may not normally be aware off but as you point out can be misscontrued.

Any DX8 users actually had issues themselves concerning poor range checks etc. I have one very reliable source who will currently not fly his DX8 because of this.

Feel free to contact me via PM if you would rather remain anonymous but there must be a few DX8 users on this forum using these by now. The cake is in the eating. Theres not much point in looking at any of the American forums due to the differences.

thanks Brian

[Geek]

To count frame and holds do you have to be running the telemetry thing on the Helli?
I ask this because I have only installed it on my 600 for a test while
A I sell the 600
B build my 700 which it will go into.

Because it's not staying in the 600 I just wrapped the telemetry Ariel around in a loop and tie wrapped it to stop it flying in to a gear or linkage etc.

So I guess the telemetry is not running very good range
saying that I have flown along way away and not seen the telemetry stop


Back to the range test My dx7 used to be bad but was returned and has been a lot better since

Me and a mate tested them side by side and mine used to be bad
about half my mates.

I like my Dx8 sorry

[jamiem]

I range checked my dx8 and it's about 28 paces on my 450 with a 6100e park fly rx. I must admit I should check it on the N5 and N9, but in flight both have been perfect. I have telemetry modules on both, and both are returning perfectly normal amounts of frame losses (both around 20 iirc on a 8 minute flight). I've also done some "dot in the sky" autos without issue.

Personally I love the DX8. It feels higher quality than the DX7, has some great features (not just the telemetry, but things like the vibration alerts, ease of servo programming, condition setting on things like expo, mixing etc), but something that has taken a bit of getting used to is the way it sits in your hand. It is a bit thicker than the 7, I think mainly due to the rubber grips, and with my pinch method it's taken a bit of experimenting to get it right. I've found that it's taken a lot of fiddling with stick lengths, and I've also now fitted a neck strap adaptor to tip the tx forward (although it balances perfectly without it). I think I've now got or nailed, and hope I can enjoy it.

[pchristy]

Barney,

For anyone having issues with 2.4 GHz, I strongly recommend getting one of these:

MetaGeek Wi-Spy 2.4i 2.4GHz Spectrum Analyzer | MetaGeek Wi-Spy

I bought one some years ago, and it has come in handy on a number of occasions!

Firstly, you can use it with a laptop to check your field for interference - there shouldn't be much on 2.4 GHz, but you never know!

Secondly, you can use it to compare transmitter outputs.

If you are doing the latter, make sure you back off far enough so that the analyzer isn't limiting at the top of its range. Place the Tx in a fixed (and repeateble!) position, and measure it. replace tranny with another (in the same position!) and measure again. The two should give very similar indications.

If you are more than about ten or fifteen feet away from the analyzer, yo can place the two trannies side by side, switch one on first, then the other, and do an immediate comparison. You can compare DSM Txs this way, and a DSM with a FASST, but not two FASSTs for reasons that become obvious when you try it!

They also do some more expensive versions that you can use with external aerials, but I just stick mine on a USB extension lead, and up a pole! Works brilliantly!

[pchristy]

Barney,

One other thought - I believe the BMFA bought a couple of WiSpy units, and that these may be available for affiliated clubs to borrow. Might be worth a call to head office?