EEVblog #1213 - The Oscilloscope Interpolation Trap - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: May 21, 2019, 01:23:48 am
content: Oscilloscope Sin X/x Interpolation can be a trap for young players, find out why.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 21, 2019, 02:40:26 am
content: It's a trap foroldplayers, too!The display at 2:40 or so, looks for all the world like that from a "2T" "sine squared pulse" when testing an analog video system with good, but not excellent, characteristics.The trap for anyone who had spent a lot of time on analog TV testing would be that they would assume that the original pulse was intended to be of that shape, "because we are used to it!" & think that the designer of the DUT had worked out some "cunning plan" to obtain correct timing/data retrieval from a signal of that shape.The other comments about sample rate -v-"time/div" is what we kept running into with early DSOs, where the sample rate would "slow down to a crawl" at settings like 5ms/div & slower, due to the tiny sample memory they possessed.

author: EEVblog
author_rank: Unknown
timestamp: May 21, 2019, 02:57:49 am
content: Quote from: vk6zgo on May 21, 2019, 02:40:26 amVery true!

author: TD-Linux
author_rank: Contributor
timestamp: May 21, 2019, 03:01:12 am
content: Another excellent video about digital sampling by Monty of Xiph, this time in respect to audio (but it's the exact same problem):https://xiph.org/video/vid2.shtmlyoutube:

author: EEVblog
author_rank: Unknown
timestamp: May 21, 2019, 03:12:37 am
content: Quote from: TD-Linux on May 21, 2019, 03:01:12 amThat's a really great video

author: bitwelder
author_rank: Super Contributor
timestamp: May 21, 2019, 10:30:40 am
content: That's not a Sin(x)/x graph, that's an incoming YT strike!

author: ggchab
author_rank: Frequent Contributor
timestamp: May 21, 2019, 11:04:28 am
content: Very interesting videoThank youSome time ago, I discovered this book that starts from the very beginning. I like it."The Scientist and Engineer's Guide to Digital Signal Processing" by Steven W. Smith (http://www.dspguide.com/)

author: Kleinstein
author_rank: Super Contributor
timestamp: May 21, 2019, 11:28:51 am
content: In the interpolated signal there is a dead give away that the ringing is due to some kind of digital filtering / interpolation: there was ringing before the transition. Ringing from a probe or poor signal quality has ringing only after the transition.

author: Fungus
author_rank: Super Contributor
timestamp: May 21, 2019, 12:09:34 pm
content: Quote from: EEVblog on May 21, 2019, 03:12:37 amShould be required viewing.(Bonus: It has lots of old test gear)

author: tautech
author_rank: Super Contributor
timestamp: May 21, 2019, 12:59:57 pm
content: Who says memory depth and sample rate are just sales gimmicks ?

author: Fungus
author_rank: Super Contributor
timestamp: May 21, 2019, 01:23:50 pm
content: Quote from: tautech on May 21, 2019, 12:59:57 pmTektronix salesmen?Rigol MSO5000 haters...?

author: EEVblog
author_rank: Unknown
timestamp: May 21, 2019, 02:22:49 pm
content: Quote from: Kleinstein on May 21, 2019, 11:28:51 amYes, i should have mentioned that.

author: Fungus
author_rank: Super Contributor
timestamp: May 21, 2019, 03:08:09 pm
content: Quote from: EEVblog on May 21, 2019, 02:22:49 pmYep.Needs emphasis: Anythingbeforethe rising edge is from the oscilloscope's interpolation, always, not from the signal or the probes. It will probably have a counterpart at the top of the rising edge, too.See:"Gibbs phenomenon"(horrible page full of math that doesn't explain much)

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: May 21, 2019, 03:19:55 pm
content: Hm, is is just me or does that red sinc drawing look a bit "pubertal"? I mean I would totally expect things like that in a school toilet but I guess some people (who don't even bother to look into the video) could be offended by this on Youtube.

author: Fungus
author_rank: Super Contributor
timestamp: May 21, 2019, 03:24:43 pm
content: Quote from: 0xdeadbeef on May 21, 2019, 03:19:55 pmIt's almost as if you haven't watched the video.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: May 21, 2019, 03:31:17 pm
content: Admittedly I was only interested in the SDS5104X part since I didn't expect to learn anything new about sinc/linear interpolation or subsampling. So I skipped over the rest of the video.I don't get the point though. I'm not offended that easily personally, I just wondered if it's such a good idea to use that kind of drawing on the intro screen since there are a lot of people who are easily offended out there.

author: Brumby
author_rank: Unknown
timestamp: May 21, 2019, 04:23:53 pm
content: As Dave said, it's "the 10 year old" in him that saw that. I didn't see it myself until after watching the video and hearing that comment from Dave.Personally, if people want to see that, then I think they are looking for it and complaining would be their childish tantrum way of making themselves feel good. Edit: I wasnotreferring to you, 0xdeadbeef!I'd have been a little happier if it hadn't been pointed out - but, hey, Dave's channel ... Dave's call. I can live with it.

author: tggzzz
author_rank: Super Contributor
timestamp: May 21, 2019, 04:51:35 pm
content: My personal preference is to use a sampling scope (digitising or analogue) set to show me the samples only.My brain can "join the dots" and interpret that information easily and, more importantly,not infer information that isn't actually there.I've come across very irritating digitising scopes allow you to select "dots only" mode, but whenever you change any control it reverts to "hide dots" interpolated mode. Grrr.

author: thinkfat
author_rank: Unknown
timestamp: May 21, 2019, 09:11:43 pm
content: Much truth is in the sentence: "before you measure, be sure about what you expect to see".

author: tggzzz
author_rank: Super Contributor
timestamp: May 21, 2019, 09:29:57 pm
content: Quote from: thinkfat on May 21, 2019, 09:11:43 pmYes.And in "trust, but verify".And in "Think" - one of only two mottos that are worth a damn, the other being "Be Prepared".

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2019, 10:34:43 pm
content: Quote from: Fungus on May 21, 2019, 03:08:09 pmThat is incorrect; preshoot can occur in the analog domain as well. For instance lumped-parameter transmission lines create preshoot when excited beyond their cutoff frequency and this is visible on some analog oscilloscopes like the 300 MHz Tektronix 2465 and following models which pushed the capability of their delay lines.It is easy enough however to distinguish this from the Gibbs phenomenon which is symmetrical around the transition.Back when DSOs were new, Tektronix had an idea for detecting the problem Dave showed.The processed trigger signal can be used to estimate the frequency content of the input signal. If it approaches or is greater than the current Nyquist frequency, (1) then it is very likely that aliasing has occurred and a warning was displayed to the user. This idea completely failed because it made it seem like Tektronix DSOs suffered from aliasing more than their competitor's DSOs, which they did not, so it was quietly dropped.I seem to recall using some DSOs which displayed a warning when interpolation was used to fill in the display. That seems like an easy solution but no doubt it suffers from the same problem. Again, marketing triumphs over engineering.(1) Or if any trigger period is less than twice the sample period.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 21, 2019, 11:37:04 pm
content: Quote from: Kleinstein on May 21, 2019, 11:28:51 amThanks Kleinstein, for the valuable tip.Would that be caused by an artifact similar to an IIR filter?EDIT : I watched the video. It explained it exactly.

author: EEVblog
author_rank: Unknown
timestamp: May 21, 2019, 11:59:18 pm
content: Quote from: 0xdeadbeef on May 21, 2019, 03:31:17 pmSo what if people are offended?Whoopty-do.

author: snoopy
author_rank: Frequent Contributor
timestamp: May 22, 2019, 05:32:34 am
content: Quote from: EEVblog on May 21, 2019, 11:59:18 pmHe doesn't understand Aussie humourLOL

author: snoopy
author_rank: Frequent Contributor
timestamp: May 22, 2019, 05:36:30 am
content: Quote from: David Hess on May 21, 2019, 10:34:43 pmEquivalent time sampling scopes don't have the same trouble with aliasing provided the signal you are observing is repetitive. Unfortunately Tek has dropped this feature on its newer scopes in favour of interpolation with all the issues of aliasing. It's a pity because they did it extremely well on the scopes that offered it.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 22, 2019, 08:59:31 am
content: Quote from: Kleinstein on May 21, 2019, 11:28:51 amIn the case of testing a limited bandwidth system, like analog TV, the signal I mentioned in my previous posting (a "T"or "2T" "sine squared pulse)*will often show both "pre shoot" & "post shoot", the amplitude of which are primarily related to the bandwidth & Group Delay of the system.Unfortunately, the artefact at 2:40 in Dave's video looks like a typical result of such testingAnother "trap for old players!"*In the early days of TV, a rectangular pulse with sharp rise & fall times was used, but massive ringing due to the limited system bandwidth caused difficulties in measurement, so the " sine squared pulse"was devised.

author: wilfred
author_rank: Super Contributor
timestamp: May 22, 2019, 09:38:22 am
content: Quote from: 0xdeadbeef on May 21, 2019, 03:19:55 pmLook at the bright side. Fewer people will fall for the interpolation trap as a result of the increased popularity of the video. I have recently become aware that catchy thumbnails is one way of fertilising the seeds of knowledge.

author: EEVblog
author_rank: Unknown
timestamp: May 22, 2019, 01:29:11 pm
content: Quote from: Brumby on May 21, 2019, 04:23:53 pmObviously the 40+ yo in me saw it too.I put out a public call on twitter the other week for the first person to get a manufacturer to silkscreen a dick'n'balls onto their chip wins The Internet.The world has gone to politically correct hell, the "easily offended" need to be ridiculed, not pandered to. Dick'n'Balls on everything.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 22, 2019, 05:17:50 pm
content: I've seen Mickey Mouse and other goofy stuff etched on chips, a politically incorrectViva Cuba Libre, would be a quantum step improvement.

author: Bud
author_rank: Super Contributor
timestamp: May 22, 2019, 06:56:04 pm
content: Quote from: EEVblog on May 22, 2019, 01:29:11 pm...and IN everything, like firmware

author: rhodges
author_rank: Frequent Contributor
timestamp: May 22, 2019, 08:22:12 pm
content: Look at the Broadcom logo...

author: vk6zgo
author_rank: Super Contributor
timestamp: May 23, 2019, 02:32:22 am
content: Quote from: Bud on May 22, 2019, 06:56:04 pmWell they dick around with,the firmware & balls it up!

author: David Hess
author_rank: Super Contributor
timestamp: May 23, 2019, 03:23:28 am
content: Quote from: snoopy on May 22, 2019, 05:36:30 amThat is true but DSOs that support equivalent time sampling usually do not at slower sweep speeds (1) where the real time sample rate will completely fill the acquisition record in one acquisition and many DSOs back then did not support equivalent time sampling at all. For a while it was a premium feature.So detecting possible aliasing by comparing the processed trigger and sample rate would still be useful even on a DSO which supports equivalent time sampling. It would be useful even with the huge record lengths available today.QuoteMost manufacturers dropped it and the reason is simple enough; the performance of integrated ADCs and memory reached a point where real time sampling rates were sufficient to support digital triggering at a cost lower than the dedicated hardware needed to measure the trigger to sampling clock delay for equivalent time sampling.Support for equivalent time sampling is still found in higher end DSOs were the cost of supporting a real time sample rate high enough for increased bandwidth is prohibitive.(1) DPO style DSOs do not care a whit about this. They effectively always operate in equivalent time mode.

author: nctnico
author_rank: Super Contributor
timestamp: January 08, 2020, 06:33:52 pm
content: Late to the game but since this video came up in another thread: the sin x/x is broken on the SDS1202X-E which is shown in the video. On any decent scope sin x/x will show a sine wave up to less than 2.5 fs (that is less than 2.5 points per period). And the frequency response on moderns DSOs doesn't roll off Gaussian. The filters are much steeper to avoid aliasing.

author: David Hess
author_rank: Super Contributor
timestamp: January 09, 2020, 02:32:38 am
content: Quote from: nctnico on January 08, 2020, 06:33:52 pmIs the Rigol DS1000Z series considered modern? Various reported tests here show that it has something close to a 6dB/octave Gaussian or Bessel response.https://www.eevblog.com/forum/testgear/rigol-ds1074z-inside-picture/msg337710/#msg337710I would only expect a steeper roll-off in a much higher bandwidth oscilloscope where a Gaussian or Bessel response is not practical.

author: nctnico
author_rank: Super Contributor
timestamp: January 09, 2020, 09:43:42 am
content: Quote from: David Hess on January 09, 2020, 02:32:38 amIt is more likely Rigol just didn't care to implement a proper anti-aliasing filter. After all it adds more parts and thus costs.

author: thinkfat
author_rank: Unknown
timestamp: January 09, 2020, 09:58:11 am
content: Quote from: nctnico on January 09, 2020, 09:43:42 amAnd it's not justified. After all, your anti-aliasing filter doesn't need to be perfect, you just need the aliasing products to be drowned in the noise of the instrument. Dynamic range is a factor, too, and and 8 bit DAC doesn't have a lot of that, either.

author: nctnico
author_rank: Super Contributor
timestamp: January 09, 2020, 10:47:59 am
content: Quote from: thinkfat on January 09, 2020, 09:58:11 amIn the frequency domain the roll-off of the (analog) anti-aliasing filter mirrors around Fs/2 so you don't need full attenuation at Fs/2. Still a simple RC filter isn't enough; a 2nd order filter is about the minimum.Look at these graphs I made on the R&S RTM3004:and the GW Instek GDS-2204E: