5 ms·

I know next to nothing about radio, can someone tell me why a baud rate limit put in to begin with? Is there a technical reason for it like causing interference

by donatj 3y ago

I know next to nothing about radio, can someone tell me why a baud rate limit put in to begin with? Is there a technical reason for it like causing interference or something?

TedDoesntTalk 3y ago
I think to limit the amount of spectrum used by any one signal… prevent bandwidth hogs.
ARandomerDude 3y ago
Uh oh. Looks like the Stack Overflow mods among us closed your question as off topic.
- donatj 3y ago
  I reworded it to be more clear what I was asking and it turned around. It was fair.
gaze 3y ago
In nearly all cases, the lower the baud rate, the lower the bandwidth. You can come up with sufficiently pathological cases but indeed it’s the bandwidth that should be limited. EDIT: you should look at the shanon-Hartley limit. The bandwidth is proportional to the symbol rate.
- Animats 3y ago
  > In nearly all cases, the lower the baud rate, the lower the bandwidth. You can come up with sufficiently pathological cases but indeed it’s the bandwidth that should be limited. This is not the case for classical frequency-shift amateur radioteletype. This sends two tones on single sideband, at 2125 Hz or 2295 Hz. So it uses up about 2.3KHz of bandwidth no matter how low the baud rate goes. 45.45 baud is classic mechanical Teletype speed, so the bandwidth is about 50x the data rate in that mode. You can do FSK up to maybe 600 baud; you need a few cycles to detect the tone frequency with classical filters. 300 is a traditional limit. Beyond that antique technology, you need a modulation scheme less than half a century old.
  - Johnythree 3y ago
    Your "2.3KHz of bandwidth" is caused by using a wide SSB filter to send a relatively narrow shift of 170Hz. You equally could have used a narrower filter.
    - Animats 3y ago
      Pre-single-sideband suppressed carrier systems used an AM carrier, so there really were two audio tones. It's also possible to send FSK over FM, which was often done on VHF where there's more room. Before everything went digital, this sort of thing tended to involve a chain of analog processing stages.
      - Johnythree 3y ago
        Hams sometimes used Audio Frequency Shift tones modulated on to an AM carrier as a temporary way of getting on air with an old style AM transmitter. Which of course meant two sets of sidebands (eg four in total) and a excessively wide signal.. But any of the better quality transmitters (Ham, Commercial or military) used pure FSK directly.
Steltek 3y ago
When the limit was put in place, we didn't know that symbol rate and baud rate could be different. A lot of these discoveries really came into their own during the 90's and enabled far more data transmission than previously imagined. However, amateur radio has some natural conservatism to it, like Morse code requirements (since retired) to gatekeep the hobby. Getting rid of this baud rate limit is long overdue as analog transmissions are laughably archaic for anything outside amateur radio.
- thrtythreeforty 3y ago
  Do you mean bit rate and baud rate can be different? My understanding is that baud rate and symbol rate are the same thing.
  - fsckboy 3y ago
    if there are two possible symbols in your communication medium that you switch between, bit rate is baud rate (with the two symbols being 0 and 1). If you have more symbols, then baud/symbol rate increases will correspond to greater bitrates increases by a factor.
    - Steltek 3y ago
      Okay, I was pretty sure I had it right the first time but my confidence is rather low since it's been a while since I read up on it.
  - Steltek 3y ago
    Oops, yeah and it's not quite my domain of expertise. In fact, that's the big draw for me: Amateur radio has been amazing for filling in knowledge gaps of low level electrical engineering details. it takes things way past the simple CPU/system models from university CS class and the Arduino DC logic hobby projects. The educational potential is really top notch.
  - mal10c 3y ago
    Baud rate and symbol rate often refer to the same thing, especially in the context of digital communications. However, they don't necessarily equate to the bit rate. In the digital realm, we're familiar with the concept of 1's and 0's, which represent binary states. When we transmit a single bit, it can be visualized as a digital line being high (for 1) or low (for 0). The rate at which this line transitions from one state to another is called the baud rate or symbol rate. To understand this further, let's consider a more advanced modulation scheme. Instead of just having two states (high and low) to represent binary bits, imagine we have four distinct states: high, medium-high, medium-low, and low. These states can represent combinations of bits as follows: high = 11, medium-high = 10, medium-low = 01, and low = 00. In this scenario, each state transition represents a symbol, and since each symbol can represent two bits, the symbol rate (or baud rate) is half of the bit rate. If you know the symbol rate and want to determine the bit rate, you'd multiply the symbol rate by the number of bits per symbol. In this example, you'd multiply the baud rate by two. Most signals rely on techniques beyond simple voltage differences though to transfer information, and that's when you delve into the world of RF theory. Instead of a discrete voltage, a sine wave is used at a particular frequency. The amplitude of the sine wave can be adjusted just like we adjusted the voltage on that line. If we want even more symbols, maybe 0000 to 1111 or bigger, we can introduce another variation to the sine wave called phase. Phase of a sine wave is just shifting it left or right, but could be visualized as two people on a race track. If they start a race from the same line and run at the same speed in the same direction, they're in phase. If one of them starts a quarter of the way ahead from the other and they both run at the same speed in the same direction, then he's a quarter phase shifted from the other. That adjustment of phase and amplitude falls into a broad category of RF modulation called QAM, and it's used in more than RF between two radios. It can also be used over Ethernet or PCIe busses. I could go on rambling for a long time on all this, but hopefully this helps answer your question.
kmbfjr 3y ago
Two reasons, avoid interference because the benchmark was a generously wide single-sideband transmission, and to avoid creating the opportunity for competition with commercial services. When these rules were adopted, a phone call to Europe was $3 per minute and a Telex as $10 for thirty words.
myth2018 3y ago
The spectrum is a limited resource. Rate limits restrict the amount of spectrum one occupies, leaving more of it for use by other operators. There are other restrictions like this. For instance, in my country, only clubs are allowed to transmit analog TV continously. Individuals can only make short transmissions. Some people argue that increasing the bandwidth would incentivize digital modes. I can understand the point, but I would rather experiment with low-bandwidth modes than to take larger portions of the spectrum with faster, potentially wasteful modes.
ryukoposting 3y ago
> Is there a technical reason for it like causing interference or something? That's correct, more or less. There's this thing called "frequency shift keying" (FSK) which was the de facto standard digital modulation technique at the time the regulations were created. With FSK, you transmit data by toggling back and forth between two frequencies. That abrupt jumping back-and-forth creates hellish interference for people trying to use nearby frequencies. Today, the obvious solution is to limit bandwidth so that there's a healthy amount of space for people to avoid that interference. However, in a (1970s/1980s) world where FSK is the only widespread modulation scheme, that wasn't really the mindset. The way they saw it was simpler: "higher baudrates demand more bandwidth, and more bandwidth creates more interference." Thus, they regulated baudrate as a means of regulating bandwidth. The impetus for the rule change is simple: we have better modulation techniques nowadays. In case I haven't bored you to death, the simplest one to explain is probably GFSK. It's similar to FSK, but instead of abruptly jumping between the two frequencies, you gradually slide back and forth between them (the "G" stands for Gaussian). GFSK requires more precise electronics, but it doesn't cause as much trouble for people trying to operate at nearby frequencies. There are countless other modulation techniques as well. Bluetooth is probably the most notable implementation of GFSK (though modern Bluetooth actually uses something else). For completeness: FSK is very useful and it's still widely used today. Your garage door opener uses it, among other things.

by donatj 3y ago

I know next to nothing about radio, can someone tell me why a baud rate limit put in to begin with? Is there a technical reason for it like causing interference or something?

TedDoesntTalk 3y ago

I think to limit the amount of spectrum used by any one signal… prevent bandwidth hogs.

ARandomerDude 3y ago

Uh oh. Looks like the Stack Overflow mods among us closed your question as off topic.

donatj 3y ago
I reworded it to be more clear what I was asking and it turned around. It was fair.

gaze 3y ago

In nearly all cases, the lower the baud rate, the lower the bandwidth. You can come up with sufficiently pathological cases but indeed it’s the bandwidth that should be limited. EDIT: you should look at the shanon-Hartley limit. The bandwidth is proportional to the symbol rate.

Animats 3y ago
> In nearly all cases, the lower the baud rate, the lower the bandwidth. You can come up with sufficiently pathological cases but indeed it’s the bandwidth that should be limited. This is not the case for classical frequency-shift amateur radioteletype. This sends two tones on single sideband, at 2125 Hz or 2295 Hz. So it uses up about 2.3KHz of bandwidth no matter how low the baud rate goes. 45.45 baud is classic mechanical Teletype speed, so the bandwidth is about 50x the data rate in that mode. You can do FSK up to maybe 600 baud; you need a few cycles to detect the tone frequency with classical filters. 300 is a traditional limit. Beyond that antique technology, you need a modulation scheme less than half a century old.
- Johnythree 3y ago
  Your "2.3KHz of bandwidth" is caused by using a wide SSB filter to send a relatively narrow shift of 170Hz. You equally could have used a narrower filter.
  - Animats 3y ago
    Pre-single-sideband suppressed carrier systems used an AM carrier, so there really were two audio tones. It's also possible to send FSK over FM, which was often done on VHF where there's more room. Before everything went digital, this sort of thing tended to involve a chain of analog processing stages.
    - Johnythree 3y ago
      Hams sometimes used Audio Frequency Shift tones modulated on to an AM carrier as a temporary way of getting on air with an old style AM transmitter. Which of course meant two sets of sidebands (eg four in total) and a excessively wide signal.. But any of the better quality transmitters (Ham, Commercial or military) used pure FSK directly.

Steltek 3y ago

When the limit was put in place, we didn't know that symbol rate and baud rate could be different. A lot of these discoveries really came into their own during the 90's and enabled far more data transmission than previously imagined. However, amateur radio has some natural conservatism to it, like Morse code requirements (since retired) to gatekeep the hobby. Getting rid of this baud rate limit is long overdue as analog transmissions are laughably archaic for anything outside amateur radio.

thrtythreeforty 3y ago
Do you mean bit rate and baud rate can be different? My understanding is that baud rate and symbol rate are the same thing.
- fsckboy 3y ago
  if there are two possible symbols in your communication medium that you switch between, bit rate is baud rate (with the two symbols being 0 and 1). If you have more symbols, then baud/symbol rate increases will correspond to greater bitrates increases by a factor.
  - Steltek 3y ago
    Okay, I was pretty sure I had it right the first time but my confidence is rather low since it's been a while since I read up on it.
- Steltek 3y ago
  Oops, yeah and it's not quite my domain of expertise. In fact, that's the big draw for me: Amateur radio has been amazing for filling in knowledge gaps of low level electrical engineering details. it takes things way past the simple CPU/system models from university CS class and the Arduino DC logic hobby projects. The educational potential is really top notch.
- mal10c 3y ago
  Baud rate and symbol rate often refer to the same thing, especially in the context of digital communications. However, they don't necessarily equate to the bit rate. In the digital realm, we're familiar with the concept of 1's and 0's, which represent binary states. When we transmit a single bit, it can be visualized as a digital line being high (for 1) or low (for 0). The rate at which this line transitions from one state to another is called the baud rate or symbol rate. To understand this further, let's consider a more advanced modulation scheme. Instead of just having two states (high and low) to represent binary bits, imagine we have four distinct states: high, medium-high, medium-low, and low. These states can represent combinations of bits as follows: high = 11, medium-high = 10, medium-low = 01, and low = 00. In this scenario, each state transition represents a symbol, and since each symbol can represent two bits, the symbol rate (or baud rate) is half of the bit rate. If you know the symbol rate and want to determine the bit rate, you'd multiply the symbol rate by the number of bits per symbol. In this example, you'd multiply the baud rate by two. Most signals rely on techniques beyond simple voltage differences though to transfer information, and that's when you delve into the world of RF theory. Instead of a discrete voltage, a sine wave is used at a particular frequency. The amplitude of the sine wave can be adjusted just like we adjusted the voltage on that line. If we want even more symbols, maybe 0000 to 1111 or bigger, we can introduce another variation to the sine wave called phase. Phase of a sine wave is just shifting it left or right, but could be visualized as two people on a race track. If they start a race from the same line and run at the same speed in the same direction, they're in phase. If one of them starts a quarter of the way ahead from the other and they both run at the same speed in the same direction, then he's a quarter phase shifted from the other. That adjustment of phase and amplitude falls into a broad category of RF modulation called QAM, and it's used in more than RF between two radios. It can also be used over Ethernet or PCIe busses. I could go on rambling for a long time on all this, but hopefully this helps answer your question.

kmbfjr 3y ago

Two reasons, avoid interference because the benchmark was a generously wide single-sideband transmission, and to avoid creating the opportunity for competition with commercial services. When these rules were adopted, a phone call to Europe was $3 per minute and a Telex as $10 for thirty words.

myth2018 3y ago

The spectrum is a limited resource. Rate limits restrict the amount of spectrum one occupies, leaving more of it for use by other operators. There are other restrictions like this. For instance, in my country, only clubs are allowed to transmit analog TV continously. Individuals can only make short transmissions. Some people argue that increasing the bandwidth would incentivize digital modes. I can understand the point, but I would rather experiment with low-bandwidth modes than to take larger portions of the spectrum with faster, potentially wasteful modes.

ryukoposting 3y ago

> Is there a technical reason for it like causing interference or something? That's correct, more or less. There's this thing called "frequency shift keying" (FSK) which was the de facto standard digital modulation technique at the time the regulations were created. With FSK, you transmit data by toggling back and forth between two frequencies. That abrupt jumping back-and-forth creates hellish interference for people trying to use nearby frequencies. Today, the obvious solution is to limit bandwidth so that there's a healthy amount of space for people to avoid that interference. However, in a (1970s/1980s) world where FSK is the only widespread modulation scheme, that wasn't really the mindset. The way they saw it was simpler: "higher baudrates demand more bandwidth, and more bandwidth creates more interference." Thus, they regulated baudrate as a means of regulating bandwidth. The impetus for the rule change is simple: we have better modulation techniques nowadays. In case I haven't bored you to death, the simplest one to explain is probably GFSK. It's similar to FSK, but instead of abruptly jumping between the two frequencies, you gradually slide back and forth between them (the "G" stands for Gaussian). GFSK requires more precise electronics, but it doesn't cause as much trouble for people trying to operate at nearby frequencies. There are countless other modulation techniques as well. Bluetooth is probably the most notable implementation of GFSK (though modern Bluetooth actually uses something else). For completeness: FSK is very useful and it's still widely used today. Your garage door opener uses it, among other things.