"Ajay" <mishraka@gmail.com> wrote in message news:1114341290.599181.144780@z14g2000cwz.googlegroups.com...> Hi, > > I am implementing a RF modulator for video signals. > [...]I'm assuming that the video is digital and the desired IF output is analog, right?> Now, Please help me come out with a technically sound and vialble > choice. If in case there exits certain other strategies too, do let me > know about them.If you'd prefer not to require too much care and expertise in analog design, you might try something like this (working from the output to the input): - Use an analog IF bandpass filter to remove the unwanted sideband, but make the filter wide enough that you don't need to be too careful with it. Add, say, 20% to the desired -60dB bandwidth. - Modulate the (real-valued) analog output from the DSP with some frequency high enough to be well suppressed by your IF filter. - In the DSP, use a sampling rate high enough to let you control the entire IF passband. If your video is critically sampled, 2x oversampling should suffice. - Do the good VSB filtering in the DSP using a complex FIR, and frequency-shift the signal so that, after modulation, the output is where you want it in the IF passband. After the frequency shift, you can discard the imaginary component of the signal. -- Matt
Hilbert Vs bandpass filter in VSB Modulation
Started by ●April 24, 2005
Reply by ●April 25, 20052005-04-25
Reply by ●April 25, 20052005-04-25
Jerry Avins wrote:> Ajay wrote:-snip-> >> o>also having two threads of operation on x[n] and hilb_x[n] will >> increase the hardware which can outweigh the advantage of using it. > > > What advantage? I can't see how to make it work. Vestigial sideband was > invented to achieve much of the benefit of single sideband while > avoiding the complexity of generating it. Modifying a method for > generating single sideband in order to produce vestigial sideband would > only add to the complexity that vestigial was designed to avoid. (In the > early days of television, the transmitters generated ordinary > double-sideband RF signals. A filter consisting of tuned stubs properly > spaced along the waveguide feeding the antenna left only a vestige of > the lower sideband and attenuated the low-frequency end of the upper > sideband, maintaining flat frequency response. I had the misfortune to > be assigned to design one of those filters. Do you know Smith charts?)If the modulator really needs to be implemented in digital, with just a DAC output to a filter and RF amplifier, then there way well be a computational advantage to using the phasing method to generate the VSB signal. If the VSB signal definition still allows for truly "vestigial" sidebands instead of "nonexistent" ones then that would presumably ease the implementation of a phasing VSB generator for a clever implementor. Of course, without expending serious effort I couldn't tell you which one will be better... -snip->>> An analog modulator, such as used to be common with video games, can >>> be had quite cheaply. Is there a reason to go digital? >> >> >> >> As the block is a pert of all digital SOC, it will be disasterous to >> have an analog RF modulator as per our case is concerned. > > > I see. > >> Moreover, is it easier to implement in analog? > > > I know it's easy and cheap: > > http://www.freepatentsonline.com/4816905.html > > http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=26-609 > > The modulator probably costs less than $2 for parts and assembly. > > http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=26-610 > > > http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=15-2526 > >Do you know for a fact that these produce VSB? My understanding has been that they just produce an AM signal and if you don't like the extra non-vestigial sideband you can just lump it. If the TV is on-channel it doesn't care if it's AM or VSB; all that an AM signal does does is interfere a bit more with the adjacent channel that it was going to trash anyway. For a $15 retail part I don't think you can expect VSB unless there are some regulations that I don't know about. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●April 25, 20052005-04-25
Tim, OK, I see your point, in the transmitter, the passband of the HT would be about 750 kHz to 4 -5 MHz (not near DC) which would be practical. thanks Also I agree with you, the cheap game type modulators are typically full DSB AM and these would create interference into the lower adjacent channel, but since games are usually connected to a TV with an antenna selector switch, for these applications, it is not an issue) Ajay, what is your application? Mark
Reply by ●April 25, 20052005-04-25
Tim Wescott wrote: ...> If the modulator really needs to be implemented in digital, with just a > DAC output to a filter and RF amplifier, then there way well be a > computational advantage to using the phasing method to generate the VSB > signal. If the VSB signal definition still allows for truly "vestigial" > sidebands instead of "nonexistent" ones then that would presumably ease > the implementation of a phasing VSB generator for a clever implementor.The VSB spec I worked to was quite specific. The response at the antenna begins one MHz below the carrier, rising linearly to 50% at the carrier and reaching 100% one MHz above it. Mark gave a very different picture, one that makes a lot of sense, but I don't know how to reconcile it with what I was taught. ... Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 25, 20052005-04-25
Jerry, I think we are in agreement, what you described above is the spec for the receiver (in North America) (if you change 1 MHz to 1.25 MHz). The (professional) TV transmitter however is flat (i.e. full DSB AM) down to -1.25 MHz then rolls off sharply, so it is DSB for modulating frequences <1.25MHz and SSB (with carrier) for modulating frequencies > 1.25 MHz. In the receiver, the detector always sees 1 sideband worth of power. For modulating frequencies >1.25 MHz it sees only the upper sideband, the lower one is not transmistted and not received. For modulating frequencies near the carrier, both sidebands are transmited, but they are on the slope of the receiver filter. The detector sees the two sidebands but they are reduced in amplitude. When it all works right, the overall frequency response is flat. (In the old days with manual tune TV's if you had a weak signal you could "mistune" the receiver so the carrier was up on the passband and enjoy somewhat reduced noise at the expense of less resolution.) The cheap game type modualtors are sometimes simple full DSB AM. The avaialbility of cheap SAW filters has changed that somewhat. Mark
Reply by ●April 26, 20052005-04-26
Mark wrote:> Jerry, > > I think we are in agreement, what you described above is the spec for > the receiver (in North America) (if you change 1 MHz to 1.25 MHz). > > The (professional) TV transmitter however is flat (i.e. full DSB AM) > down to -1.25 MHz then rolls off sharply, so it is DSB for modulating > frequences <1.25MHz and SSB (with carrier) for modulating frequencies > > 1.25 MHz.Yes, 1.25 MHz. <chagrin> Is it mitigating that I last dealt with this in 1954?> In the receiver, the detector always sees 1 sideband worth of power. > For modulating frequencies >1.25 MHz it sees only the upper sideband, > the lower one is not transmistted and not received. For modulating > frequencies near the carrier, both sidebands are transmited, but they > are on the slope of the receiver filter. The detector sees the two > sidebands but they are reduced in amplitude. When it all works right, > the overall frequency response is flat.I believed you. I'm puzzled that they have me designing waveguide filters that could withstand 50 KW without flashing over. A sharp cutoff would have been easier than that 2.5-MHz-wide linear slope.> (In the old days with manual tune TV's if you had a weak signal you > could "mistune" the receiver so the carrier was up on the passband and > enjoy somewhat reduced noise at the expense of less resolution.)And by sufficiently reducing the carrier amplitude, one could make the image go negative, like darkfield microscopy. One deeded a set with intercarrier sound, to play those tricks and still hear.> The cheap game type modualtors are sometimes simple full DSB AM. The > avaialbility of cheap SAW filters has changed that somewhat.Thanks. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 26, 20052005-04-26
In my application, passband has to be from -5Mhz to 750KHz. As it is easier to get better performance from filters at low frequency, I am willing to stick with Hilbert option. however, normally hilbert transfrom does 90 phase shift for all postive frequencies and -90 for negative ones. I want it to do 90 phase shift for frequencies above 750Khz - 5MHz and -90 degree phase shift for -750KHz to -5MHz. This will create two components which on combining would elide upper sideband starting from 750KHz. I want your help in desiging such a Hilbert Transformer. Please let me know if something is still not clear. -Ajay
Reply by ●April 26, 20052005-04-26
Mark wrote:> Jerry, > > I think we are in agreement, what you described above is the spec for > the receiver (in North America) (if you change 1 MHz to 1.25 MHz). > > The (professional) TV transmitter however is flat (i.e. full DSB AM) > down to -1.25 MHz then rolls off sharply, so it is DSB for modulating > frequences <1.25MHz and SSB (with carrier) for modulating frequencies > > 1.25 MHz.I wonder how can it happen. If we have got baseband video signal whose spectrum ranges from -5Mhz to 1.25Mhz, for all modulating frequencies, it will remain unsymmetric spectrum centred at the modulating frequencies. Correct if I am wrong. So it should remain DSB you prefer to call it instead of VSB.> The cheap game type modualtors are sometimes simple full DSB AM. The > avaialbility of cheap SAW filters has changed that somewhat.how difficult is to design SAW filter for bandpass frequencies like 61.25Mhz?? -Ajay
Reply by ●April 26, 20052005-04-26
I don't understand your question. Baseband video is a real signal from DC to about 4.2 MHz. When applied to a DSB AM modulator, it creates symmetrical sidebands. Filtering removes part of the lower sideband (at RF, at IF the upper sideband is removed). Alternatively the phasing method can be used to remove part of the unwanted (vestigial) sideband. Its easy for a SAW designer to design the required SAW filter difficult for me :-) ) I think the process is similar to designing an FIR filter. There is a large selection of IF and also Ch 3 and Ch 4 SAWs commercially available for low cost modulators. There are also high quality expensive ones. What is your application? Is it a low cost consumer device for connection to a single TV or a high quality CATV or broadcast device? Mark
Reply by ●April 26, 20052005-04-26
Ajay wrote:> In my application, passband has to be from -5Mhz to 750KHz. As it is > easier to get better performance from filters at low frequency, I am > willing to stick with Hilbert option. however, normally hilbert > transfrom does 90 phase shift for all postive frequencies and -90 for > negative ones. I want it to do 90 phase shift for frequencies above > 750Khz - 5MHz and -90 degree phase shift for -750KHz to -5MHz. This > will create two components which on combining would elide upper > sideband starting from 750KHz. > > I want your help in desiging such a Hilbert Transformer. Please let me > know if something is still not clear. > > -AjayHTs can also be designed by filter-design programs. I can show you how to do it by hand, but you'd be better off with a book. Unlike a sync, the function is antisymmetric but monotonic on each side of center. Odd-length filters are a bit easier to explain. Number the center coefficient h[0], and set it and all even numbered coefficient equal to zero. For every odd coefficient n, make its value 1/n, including sign. The Gibbs phenomenon will cause heavy ripple, extending well toward the center. A Blackman, Nuttall, or other "strong" window will flatten the response at the expense of bandwidth. Add more terms to restore the bandwidth. For unity gain, you will need a factor of 2/pi. The signal at tap zero is a replica of the input delayed as much as the quadrature that results from the convolution. Take all this with a grain of salt. It's all from memory and it's been years since I did an HT the hard way. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
