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README

@@ -55,74 +55,80 @@ more than anything. They may or may not work for you.
 
 First some background for those unfamiliar with the internals of the dongle:
 
- * The R820T tuner has a tunable PLL that can generate frequencies between around
-   27MHz .. 1850MHz. The exact range varies from dongle to dongle.
- * The tuner mixes the incoming RF signal with this PLL output. This shifts the
-   RF signal down to a frequency that is the difference between the PLL frequency
-   and the RF signal's frequency.
+ * The R820T tuner has a tunable PLL that can generate frequencies between
+   around 27MHz .. 1850MHz. The exact range varies from dongle to dongle.
+ * The tuner mixes the incoming RF signal with this PLL output. This shifts
+   the RF signal down to a frequency that is the difference between the PLL
+   frequency and the RF signal's frequency.
  * The tuner sends this intermediate frequency (IF) signal to the RTL2838U
- * The RTL2838U digitizes the IF signal and does a digital downconversion step to
-   generate I/Q samples that are centered around "zero". The downconverter can
-   theoretically handle IF signals up to 14.4MHz.
+ * The RTL2838U digitizes the IF signal and does a digital downconversion
+   step to generate I/Q samples that are centered around "zero". The
+   downconverter can theoretically handle IF signals up to 14.4MHz.
 
 The main change is to feed information about the actually-tuned frequency back
-from the tuner (R820T) code to the core (RTL2832U) code, allowing the core code
-to adjust to the actual IF in use, rather than requiring a fixed IF. The core code
-has also been taught how to handle low-side mixing, where the PLL frequency is lower
-than the requested frequency (e.g. the spectrum inversion changes then)
+from the tuner (R820T) code to the core (RTL2832U) code, allowing the core
+code to adjust to the actual IF in use, rather than requiring a fixed IF. The
+core code has also been taught how to handle low-side mixing, where the PLL
+frequency is lower than the requested frequency (e.g. the spectrum inversion
+changes then)
 
-When tuning, the R820T tuner will try to tune to the requested frequency + 6MHz,
-producing a 6MHz IF. If the PLL cannot handle that frequency, the tuner will try
-a few things to produce something useful.
+When tuning, the R820T tuner will try to tune to the requested frequency +
+6MHz, producing a 6MHz IF. If the PLL cannot handle that frequency, the tuner
+will try a few things to produce something useful.
 
 At the top of the tuning range:
 
- * It will tune as high as it can go normally, then stick there producing a smaller effective
-   IF as the requested frequency gets higher and higher.
- * Once the IF starts getting too small (below about 1.5MHz things start to go bad), it will
-   switch to low-side mixing and try to put the PLL frequency *below* the target frequency.
-   This gives you another 14-15 MHz at the top of the range.
+ * It will tune as high as it can go normally, then stick there producing a
+   smaller effective IF as the requested frequency gets higher and higher.
+ * Once the IF starts getting too small (below about 1.5MHz things start to
+   go bad), it will switch to low-side mixing and try to put the PLL frequency
+   *below* the target frequency. This gives you another 14-15 MHz at the top
+   of the range.
 
-I've had reports that some dongles can tune to within a whisker of 2000MHz with this.
+I've had reports that some dongles can tune to within a whisker of 2000MHz
+with this.
 
 At the bottom of the range:
 
- * It will tune as low as it can go normally, then stick there producing a larger effective IF
-   as the requested frequency gets lower and lower.
- * Once the required IF exceeds 14.4MHz, it will switch to a variant of tejeez' et al "no mod
-   direct sampling" mode. This essentially disables the PLL in the tuner entirely, and relies
-   on some RF signal leaking through the tuner unchanged. The tuner claims to be tuned to 0Hz
-   in this mode, and the core does all the real tuning. The dongle is almost deaf in this
-   mode, you will have to turn the RF gain WAY UP. You do not need to mess with direct sampling
+ * It will tune as low as it can go normally, then stick there producing a
+   larger effective IF as the requested frequency gets lower and lower.
+ * Once the required IF exceeds 14.4MHz, it will switch to a variant of tejeez'
+   et al "no mod direct sampling" mode. This essentially disables the PLL in
+   the tuner entirely, and relies on some RF signal leaking through the tuner
+   unchanged. The tuner claims to be tuned to 0Hz in this mode, and the core
+   does all the real tuning. The dongle is almost deaf in this mode, you will
+   have to turn the RF gain WAY UP. You do not need to mess with direct sampling
    settings to enable this, it's integrated with normal tuning.
    
-The success of the "no mod direct sampling" mode seems to vary a lot from dongle to dongle. Mine
-is almost completely deaf so I can't test this much. Others have had success in tuning to
-shortwave stations as low as 3.5MHz.
+The success of the "no mod direct sampling" mode seems to vary a lot from dongle
+to dongle. Mine is almost completely deaf so I can't test this much. Others have
+had success in tuning to shortwave stations as low as 3.5MHz.
    
-The actual PLL limits vary from dongle to dongle, and from day to day (they're probably
-temperature related). The tuner has three sets of tuning limits:
+The actual PLL limits vary from dongle to dongle, and from day to day (they're
+probably temperature related). The tuner has three sets of tuning limits:
 
- * a hardcoded "do not exceed this" set of limits - see PLL_INITIAL_LOW / PLL_INITIAL_HIGH in
-   tuner_r82xx.c. These are in place because, especially at the low end of the range, the PLL
-   can get into a state where you ask for (for example) 25MHz, the PLL claims to be locked OK
-   at that frequency, but in reality it's actually producing 27MHz, which screws up the
-   core's calculations of the IF offset needed.
- * a hardcoded "known to be OK" set of limits - see PLL_SAFE_LOW / PLL_SAFE_HIGH in
-   tuner_r82xx.c. This is a range where the PLL should always work; if the PLL doesn't work in
-   this range it is treated as a tuning error and tuning fails.
- * a runtime "seems to be OK at the moment" set of limits. This varies from run to run and
-   initially starts at the "do not exceed" limits. Whenever a failure to get PLL lock is seen,
-   the runtime limits are shrunk accordingly and we try again. This allows the tuner to adapt
-   to the particular dongle in use.
+ * a hardcoded "do not exceed this" set of limits - see PLL_INITIAL_LOW /
+   PLL_INITIAL_HIGH in tuner_r82xx.c. These are in place because, especially at
+   the low end of the range, the PLL can get into a state where you ask for (for
+   example) 25MHz, the PLL claims to be locked OK at that frequency, but in
+   reality it's actually producing 27MHz, which screws up the core's calculations
+   of the IF offset needed.
+ * a hardcoded "known to be OK" set of limits - see PLL_SAFE_LOW / PLL_SAFE_HIGH
+   in tuner_r82xx.c. This is a range where the PLL should always work; if the PLL
+   doesn't work in this range it is treated as a tuning error and tuning fails.
+ * a runtime "seems to be OK at the moment" set of limits. This varies from run
+   to run and initially starts at the "do not exceed" limits. Whenever a failure
+   to get PLL lock is seen, the runtime limits are shrunk accordingly and we try
+   again. This allows the tuner to adapt to the particular dongle in use.
    
-Remember that at the extreme edges of these ranges you will be fighting an uphill battle against
-the filters etc. of the dongle, as they're way out of their intended ranges and really will not
-work very well.
+Remember that at the extreme edges of these ranges you will be fighting an uphill
+battle against the dongle design itself, as you will be far out of the intended
+range of the filters etc and nothing works particularly well.
 
-You are likely to see small frequency shifts (maybe 1-2ppm) from what you get in upstream. This
-is because the tuner+2832 combination can be tuned a little more accurately than upstream does,
-so some errors that you will have previously been correcting for will disappear.
+You are likely to see small frequency shifts (maybe 1-2ppm) from what you get in
+upstream. This is because the tuner+2832 combination can be tuned a little more
+accurately than upstream does, so some errors that you will have previously been
+correcting for will disappear.
 
 
 -- Oliver (oliver.jowett@gmail.com / oliver@mutability.co.uk)