Post by therockstation on Mar 29, 2011 9:08:26 GMT -6
Hi to all, am new here, but not to broadcasting and radio. I've been reading some of the messages on here and all ot of misinformation is out there about the standards in AM and FM broadcasting, So I thought I would try to give more detailed information about each service as it applies to the US and North America, I have included some info about Europe too.
AM has a bandwidth of 10 KHz, 9 KHz in UK and 5KHz on shortwave. Audio Frequency response will always be 1/2 of the bandwidth, since each side carries the same information, 50 - 5000 Hz +-3 db. S/N Ratio will be at best 60 DB or lower. THD for AM can be as high as 5% most are 1% SSB although not normally used in broadcasting uses 1/2 of the bandwidth, though it's frequency response is the full bandwidth of the signal, usually 5kHz. FM uses a different means of using bandwidth, there are two versions of FM Narrow band, typically used in communications, of 25KHz or 10 KHz bandwidth and FM Broadcast Band which uses 150 KHz of bandwidth. There is a 25KHz guard-band on each side of the Carrier, hence 200 KHz channel spacing, You will find in any specification of Wide Frequency Modulation (WFM) the modulation value of +- 75KHz. Now here is the real important part of the FM audio sound quality that seems to be overlooked, and is very critical for great sound, Pre-Emphasis: Standard 75usec (FCC) North America Optional 50usec (CCIR) Europe
Warning many of the Europe FM Transmitters are not switchable Pre-Emphasis so be careful or your high end frequencies will be buried so either you'll need extra EQ on the high end or you'll need to go to the transmitter and replace some capacitors and resistors to get the right sound. FM has superior sound quality Frequency response 20 - 15 KHz Higher if RDS is used S/N Ratio 75 - 80+ db THD 0.3 or lower Frequency stability: +- 10ppm FCC standard is +- 200 Hz Frequency Drift will PLL is pretty much a thing of the past however with most receivers a drift of up to 1KHz will not effect performance since FM changes the frequency thousands of times per second. Spurious/Harmonic rejection: Harmonics: >50dB, Spurious: -60dB This is important.... the lower the number the better, if it's around -30 send it back or you'll be getting phone calls from the FCC for interfering TV station signals, or from your neighbors, especially if they are using a TV antenna to receive their signal. You can either build or buy FM Low Pass Filter to correct this or just be safe and get one connected so you don't have any issues, just get r done.
I read on some posts too that AM uses Pre-Emphasis this is not true, as far the analog signal is concerned,there is not enough bandwidth for the carrier to handle the AM Signal, This may not apply to the digital HD Signals of those who are broadcasting in those services. If you are one of those who are using HD AM then your losing 50% of the analog power!
Hopefully this will help the newbies and those who have been looking for information on transmitters.
I read on some posts too that AM uses Pre-Emphasis this is not true, as far the analog signal is concerned,there is not enough bandwidth for the carrier to handle the AM Signal
The FCC has adopted the NRSC broadcast standard some time ago. It is a form of pre-emphasis on the transmitters end and de-emphasis on most newer built AM radios. This is why when broadcasting bare bones using a simple AM transmitter the fidelity sounds squashed on the high end. I suggest looking up the NRSC AM broadcast standards as it is not new and has been in use by all licensed AM mediumwave stations in the US. I am not sure about other countries though. So yes AM radio in the US does indeed use pre-emphasis. The curve is around +1db at 1kHz rising to +10db at 10kHz on the transmitters end. For the receivers end it's the opposite, -1db at 1kHz lowering to -10db at 10kHz.
In the US 10kHz is the maximum passable high frequency range allowed by FCC rules. After 10kHz the curve is a hard brick wall so that audio is attenuated greatly afterwards.
The bandwidth for AM broadcasting is 20.4kHz, not 10kHz like most believe. 10.2kHz is the maximum allowed audio frequency bandwidth.
The limitation on AM fidelity comes from current receiver design. Moreover, to fit more transmitters on the AM broadcast band in the United States maximum, transmitted audio bandwidth is limited to 10.2 kHz by a National Radio Systems Committee (NRSC) standard adopted by the FCC in June 1989, resulting in a channel occupied bandwidth of 20.4 kHz. The former audio limitation was 15 kHz resulting in a channel occupied bandwidth of 30 kHz.
So there is no reason that high fidelity audio can't be broadcasted on AM. In fact using AM stereo C-QUAM it's possible to achieve fidelity near FM broadcasts. For part 15 stations there is no reason they can't run a full 15kHz of audio if they want and that would be as good of audio fidelity as FM broadcasts. There is technically NO reason AM can't sound as good, if not better than FM broadcasts. In fact the very method of amplitude modulation allows for less audio distortion than frequency modulation so in theory AM could actually sound better than FM. It's the limitations of the persons radio, and the limitations of the broadcast, especially the legal limitations set by the FCC
BTW welcome to the forums. I always look forward to these kind of discussions
You'll notice that the curve goes way down after 3 KHz it is 3 db down meaning that it is only 50% of the signal, you'll notice that the chart goes to 10K but it's at 10th the power as well. 50-60 db SNR is nothing to brag about.
FM frequency response st.gsmarena.com/vv/reviewsimg/nokia-6600-fold/gsmarena_a001.png a typical response curve for a tuner is within 3 db from 20 Hz to 15 KHz and SNR of at least 70db+ Just remember that no matter the quality of the transmitter, your only going to sound as good as your receiver and speakers in the case of AM broadcasting. More bad news for AM Stereo Broadcasters:
Technological obsolescence C-QUAM is incompatible with IBOC[vague] digital radio. The IBOC (In Band On Channel) system allows transmission of an audio frequency range extending to approximately 15 kHz, 2-ch Stereo on the AM band, but with significant digital artifact and aliasing due to substantial codec inadequacy, which is further complicated due to proprietary nature of the company, iBiquity, which not only sells and licenses the associated equipment to broadcasters, but also maintains control over its deployment, use, and distribution. This includes licensing fees for both the use of the technology, and any modifications to be made, even if the broadcaster in question has purchased the equipment outright and made costly modifications to their transmitter plant in order to implement it.
A very few AM radio stations that broadcast with IBOC HD Radio during the day switch to C-QUAM AM Stereo during nighttime operation to reduce sideband digital (hash) interference and to provide long-range stereo reception. Many HD radio tuners have a limited ability to decode C-Quam stereo transmissions, i.e. at lower bandwidth, and as a result, reduced audio quality than what could be expected from a specifically designed C-Quam only tuner.
I would say it is way beyond the common person who wishes to broadcast on the AM band with good audio quality, it would be next to impossible to get such equipment, unless you have the money or know or work for the manufacture of such equipment.
This is one of the problems with the US standard of HD Radio hybrid digital systems, they should have just made a Digital Only spec tum space from 76.1 - 84.9 MHz and allow LP-FM stations from 85.1 - 87.3 MHz. Today we have the techology to have several hundred stations with either 24 KHz bandwidth or 48 KHZ Bandwidth with mutiple streams per station on either bandwidth. Think about the narrow band FM and how far it gets out with just a few hundred milliwatts, a broadcast station of just 1 KW would be over 3 KW or over 6 KW, and that would be 100% Digital signal not the current standard of 4% of the IBOC total power a 50 KW station is only 2 KW... minus feed loss. The FCC limited the audio response and quality with it's restrictions and for selling out to a IBOC Fake digital radio of both AM & FM. it should have been an open source standard, that could be upgradeable by using a firmware updatable chip technology, much like what hardware manufactures do for just about anything from DVD recorders to TVs.
Thanks Darklife, It's always fun discussing topics like this.
Yes that response curve is based on NRSC curve, but only one half of it. Remind you that the whole point of pre/de-emphasis is to remove as much possible high frequency interference artifacts from a static filled broadcast as possible. That curve you show is the outcome at the listeners radio end. The radio station following NRSC rules will make up for that by increasing the same decreased range on the receivers part. For FM is a different story but I am still caught up trying to understand why you think AM broadcasts don't use pre-emphasis when it's a common mode of operation on AM radio stations over here.
SW is another story. They do indeed use 5kHz bandwidth for audio (10kHz bandwidth for carrier+modulation). People forget that modulation does splatter on to other stations, however the FCC set the limit for AM MW broadcasts at roughly 10kHz. Though this may splatter onto other stations a channel away in reality the figures show that high frequencies though overlap aren't of practical interference at a distance from the station and their counterparts on adjacent frequencies. FM on the other hand is another story because of the "capture effect" of FM in general and the distance of FM is not regularly utilized to the fullest extent even with RDS and subcarrier channels.
It's a science really and believe me AM or FM, it doesn't matter the mode of modulation. In reality to our ears using the right modulation/demodulation gear we humans can't tell a difference. The problem comes in with rules and regulations, crappy radios, and even more crappy broadcast outlets.
Shortwave is no different, but because of the endless clutter they are limited to 5kHz audio. 10kHz bandwidth in total carrier space modulated. But like I said, splatter is accepted at general range, and is a good thing since local stations are more important, but that splatter exponentially decreases at the linear factor of broadcast range, which is why the powers that be like the FCC allows 10kHz AM, and 5kHz SW.
FM is another whole topic. 200kHz is the bandwidth of a product of an FM legalized FM station in the US. Most of that extra space is filled with extra carrier product. Some stations utilize that with sub-carriers which they broadcast out to listeners in the know, like local stores that need basic PA music. Also a large part of that carrier is used up by the double sideband product to create stereo, and the pilot.
This is a complicated topic but I believe getting the facts right first is of ultimate value. People get mixed up easily thinking AM radio is limited to 5kHz audio, usually confused with telephone communications and standards which computer software has notoriously led people to confuse the two.
Distortion factors are another dilemma because people think because their pocket AM radio sounds like shit that all AM radio sounds like garbage. The science is in and it states that amplitude modulation does include less distortion compared to FM. Regardless of radio period this difference of modulation states that AM can sound better than FM. The problem is with bandwidth of AM radio and selectivity. With AM radios we pay the price for selectivity over audio. That single handedly is the reason why AM doesn't sound as good as FM today. However that could all change if the FCC wouldn't be a bunch of pricks and just open up the lower japan bands to US usage and let us broadcast (legal and not) on the 76-88mhz band. That would include higher bandwidth and the ability to utilize spectrum not used currently by a thousand stations on the same damn channel while all covering over eachother like a thousand voices speaking.
FM does have the fidelity, but it was set that way, not that way by default. Some people, older generations than I remember when 15kHz AM was the standard and there was no IBLOCK (IBOC digital, lol) and radio was something of fidelity. Back when music was played.
But okay we have FM today. FM is great, but it has its limitations also and to deny that is ignoring the obvious.
The obvious being we are forgetting a basic point, AM and FM modulation methods can produce quality fidelity. The limitations exist not in the method, but the radios, the transmitter. And at one time AM rivaled FM in audio quality. There is no reason that can't still be the same. But my point here is, don't get the idea that one is better than the other, the real restrictions is governed by the radio police.
Post by therockstation on Mar 29, 2011 21:52:56 GMT -6
Well ok I was wrong about the AM not using pre-emphasis, I don't know I must have had a brain freeze, Fod knows it's been winter here too long.... But I did find the document about it for anyone else who might be interested in it, there is a 10 KHz limit though, as you were stated before about the audio response.
If you really want to hear some good AM stations then I would direct everyone to the 160 Meter Band of Ham Radio, and to a lesser extend the 80 Meter band too. You will find many Elmers who operate modified AM Plate Modulated Broadcast band transmitters., with some plate modulated tube microphones. The sound is so sweet, especially on a good Ham radio with tubes, they are getting harder to find. I use a compressor with some AX-7 tubes to curb out the digital harshness, it does give your music and voices a nice warm sound.
Old thread..sort of..but I have to sound off on this AM vs FM audio debate when it comes to AM Stereo transmission.
For starters, AM has ALWAYS been limited in audio response due to the way the receivers are made. Wide-band AM receivers can show you exactly what kind of frequency response you can get from a mono AM transmission without the NRSC curve. Now throw in C-QUAM...(outdated perhaps, but still better than IBOC) and you got something to make even the most detailed audio buff guess wrong when asked "are you hearing an FM Stereo signal or an AM Stereo signal?".
FM is also limited in deviation on both ends, xmit and receive. Try deviating your FM beyond the typical 200khz IF filter in all receivers and all you end up with is spread spectrum like noise. AM however, you can pump that sucker beyond 150% positive peak and actually increase your carrier power, in turn extending range, whereas FM..you "squish" the carrier and make it weaker if attempting to over-deviate (over modulate). This can also happen when trying to shove more audio frequency response through the window, and in most cases, throw off the carrier frequency locking circuitry (PLL) or end up blowing finals or causing arching in your feed line or within the final amp itself.
160 meter band? Where are you gonna find hi-fi audio on 160? Oh...those narrow band transmissions with voice response to that of 5khz telephone audio! I suppose that could be hi-fi to aged narrowed hearing response curves.
Actually I have modified a few BC band units over to ham bands. Too much box for the purpose IMO. They mostly "look" nice all lit up with meters moving and tubes glowing. Great conversation pieces especially when the xyl throws the skillet at you over the electric bill and states "it's cheaper to simply call these people or text message them!"
Seriously...AM can do some serious kick butt to FM when done properly. IBOC.....that should have never happened...and I would be the first to take it out back and set it up on the target range.
AM Stereo 1670
FM Stereo 92.1
IBOC.....that should have never happened...and I would be the first to take it out back and set it up on the target range.
Trust me you wouldn't be the first to at least feel that way. Most professional engineers who install and maintain equipment for licensed AM stations even agree that IBOC is crap. Most people even kid around by nicknaming it "IBLOCK" because the damn digital hash splatters all over stations adjacent to the IBOC stations. It gets real bad at night when trying to do some mediumwave DXing. The stations that run IBOC are forced to increase their transmitter power to be equivalent to the power they had when not running IBOC because the high frequency digital modulation causes the carrier to widen up to 50kHz instead of the standard 10kHz bandwidth, and as you already know it makes stations to the left and right of the IBOC station almost impossible to listen to. The fact that the FCC ever even allowed this shows their stupidity and how they get bought off instead of actually caring about radio. Besides all of that IBOC doesn't sound all that good for all the hype it received.
The fact that the FCC ever even allowed this shows their stupidity and how they get bought off instead of actually caring about radio.
The radio spectrum has been turned into nothing more than chunks of real estate land lots auctioned to the highest bidder....with the Fat Crooked Cat at the top reaping all the rewards. There is no such thing as "public interest". That is only a layer of vanilla icing on the cow patty cake.
Other side of the coin..if the public had any real true interest, there would be a massive outcry for changing it. We cant put all the blame on big brother when they are allowed to do whatever they wish with no checks and balances, part of which is the public's responsibility under the constitution. A passive public only brings possessive government.
But as long as the mtv and 16 & pregnant dancing with the runs come on the screen on time, the text messages sent to the person 3 feet away, and the lady gag-gag fills the playlists, and it can all be had for 99 cents or for free...the public really does not care, or know the difference between top quality radio and cookie cutter radio.
Our task is to draw the attention of the public in the right direction. As many have stated, a digital signal or even analog signal with processing out the arse makes no difference when the programing is like the vanilla icing cow patty.
IBOC, IBLOCK, IBLOTCH, IBLAH. Corporate buyouts instead of competition, the selling out of industrial backbone, is it any wonder why the "one size fits all" genre works to the big wig's advantage.
The show must go on....over and over, round and round. Feeding the hungry with a ration pack of vanilla icing cow patty becomes tasty after repetitive shoving down the throat.
AM Stereo 1670
FM Stereo 92.1